Add connPool implementation and connection pool errors

Introduce connPool struct and implement the Pool interface. Add error handling for invalid pool capacity settings and provide a constructor for creating new connection pools with specified capacities.

.github/workflows/codecov.yml

@@ -36,7 +36,7 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-latest, macos-latest, windows-latest]
-        go: ['1.20', '1.21', '1.22', '1.23']
+        go: ['1.19', '1.20', '1.23']
     steps:
     - name: Harden Runner
       uses: step-security/harden-runner@91182cccc01eb5e619899d80e4e971d6181294a7 # v2.10.1

.github/workflows/codeql-analysis.yml

@@ -54,7 +54,7 @@ jobs:
 
     # Initializes the CodeQL tools for scanning.
     - name: Initialize CodeQL
-      uses: github/codeql-action/init@4dd16135b69a43b6c8efb853346f8437d92d3c93 # v3.26.6
+      uses: github/codeql-action/init@294a9d92911152fe08befb9ec03e240add280cb3 # v3.26.8
       with:
         languages: ${{ matrix.language }}
         # If you wish to specify custom queries, you can do so here or in a config file.
@@ -65,7 +65,7 @@ jobs:
     # Autobuild attempts to build any compiled languages  (C/C++, C#, or Java).
     # If this step fails, then you should remove it and run the build manually (see below)
     - name: Autobuild
-      uses: github/codeql-action/autobuild@4dd16135b69a43b6c8efb853346f8437d92d3c93 # v3.26.6
+      uses: github/codeql-action/autobuild@294a9d92911152fe08befb9ec03e240add280cb3 # v3.26.8
 
     # ℹ️ Command-line programs to run using the OS shell.
     # 📚 https://git.io/JvXDl
@@ -79,4 +79,4 @@ jobs:
     #   make release
 
     - name: Perform CodeQL Analysis
-      uses: github/codeql-action/analyze@4dd16135b69a43b6c8efb853346f8437d92d3c93 # v3.26.6
+      uses: github/codeql-action/analyze@294a9d92911152fe08befb9ec03e240add280cb3 # v3.26.8

.github/workflows/scorecards.yml

@@ -75,6 +75,6 @@ jobs:
 
       # Upload the results to GitHub's code scanning dashboard.
       - name: "Upload to code-scanning"
-        uses: github/codeql-action/upload-sarif@4dd16135b69a43b6c8efb853346f8437d92d3c93 # v3.26.6
+        uses: github/codeql-action/upload-sarif@294a9d92911152fe08befb9ec03e240add280cb3 # v3.26.8
         with:
           sarif_file: results.sarif

client.go

@@ -787,3 +787,100 @@ func (c *Client) auth() error {
 	}
 	return nil
 }
+
+// sendSingleMsg sends out a single message and returns an error if the transmission/delivery fails.
+// It is invoked by the public Send methods
+func (c *Client) sendSingleMsg(message *Msg) error {
+	if message.encoding == NoEncoding {
+		if ok, _ := c.smtpClient.Extension("8BITMIME"); !ok {
+			return &SendError{Reason: ErrNoUnencoded, isTemp: false, affectedMsg: message}
+		}
+	}
+	from, err := message.GetSender(false)
+	if err != nil {
+		return &SendError{
+			Reason: ErrGetSender, errlist: []error{err}, isTemp: isTempError(err),
+			affectedMsg: message,
+		}
+	}
+	rcpts, err := message.GetRecipients()
+	if err != nil {
+		return &SendError{
+			Reason: ErrGetRcpts, errlist: []error{err}, isTemp: isTempError(err),
+			affectedMsg: message,
+		}
+	}
+
+	if c.dsn {
+		if c.dsnmrtype != "" {
+			c.smtpClient.SetDSNMailReturnOption(string(c.dsnmrtype))
+		}
+	}
+	if err = c.smtpClient.Mail(from); err != nil {
+		retError := &SendError{
+			Reason: ErrSMTPMailFrom, errlist: []error{err}, isTemp: isTempError(err),
+			affectedMsg: message,
+		}
+		if resetSendErr := c.smtpClient.Reset(); resetSendErr != nil {
+			retError.errlist = append(retError.errlist, resetSendErr)
+		}
+		return retError
+	}
+	hasError := false
+	rcptSendErr := &SendError{affectedMsg: message}
+	rcptSendErr.errlist = make([]error, 0)
+	rcptSendErr.rcpt = make([]string, 0)
+	rcptNotifyOpt := strings.Join(c.dsnrntype, ",")
+	c.smtpClient.SetDSNRcptNotifyOption(rcptNotifyOpt)
+	for _, rcpt := range rcpts {
+		if err = c.smtpClient.Rcpt(rcpt); err != nil {
+			rcptSendErr.Reason = ErrSMTPRcptTo
+			rcptSendErr.errlist = append(rcptSendErr.errlist, err)
+			rcptSendErr.rcpt = append(rcptSendErr.rcpt, rcpt)
+			rcptSendErr.isTemp = isTempError(err)
+			hasError = true
+		}
+	}
+	if hasError {
+		if resetSendErr := c.smtpClient.Reset(); resetSendErr != nil {
+			rcptSendErr.errlist = append(rcptSendErr.errlist, resetSendErr)
+		}
+		return rcptSendErr
+	}
+	writer, err := c.smtpClient.Data()
+	if err != nil {
+		return &SendError{
+			Reason: ErrSMTPData, errlist: []error{err}, isTemp: isTempError(err),
+			affectedMsg: message,
+		}
+	}
+	_, err = message.WriteTo(writer)
+	if err != nil {
+		return &SendError{
+			Reason: ErrWriteContent, errlist: []error{err}, isTemp: isTempError(err),
+			affectedMsg: message,
+		}
+	}
+	message.isDelivered = true
+
+	if err = writer.Close(); err != nil {
+		return &SendError{
+			Reason: ErrSMTPDataClose, errlist: []error{err}, isTemp: isTempError(err),
+			affectedMsg: message,
+		}
+	}
+
+	if err = c.Reset(); err != nil {
+		return &SendError{
+			Reason: ErrSMTPReset, errlist: []error{err}, isTemp: isTempError(err),
+			affectedMsg: message,
+		}
+	}
+	if err = c.checkConn(); err != nil {
+		return &SendError{
+			Reason: ErrConnCheck, errlist: []error{err}, isTemp: isTempError(err),
+			affectedMsg: message,
+		}
+	}
+	return nil
+}

client_119.go

@@ -7,111 +7,23 @@
 
 package mail
 
-import "strings"
+import "errors"
 
 // Send sends out the mail message
 func (c *Client) Send(messages ...*Msg) error {
-	if cerr := c.checkConn(); cerr != nil {
-		return &SendError{Reason: ErrConnCheck, errlist: []error{cerr}, isTemp: isTempError(cerr)}
+	if err := c.checkConn(); err != nil {
+		return &SendError{Reason: ErrConnCheck, errlist: []error{err}, isTemp: isTempError(err)}
 	}
 	var errs []*SendError
-	for _, message := range messages {
-		message.sendError = nil
-		if message.encoding == NoEncoding {
-			if ok, _ := c.smtpClient.Extension("8BITMIME"); !ok {
-				sendErr := &SendError{Reason: ErrNoUnencoded, isTemp: false}
-				message.sendError = sendErr
-				errs = append(errs, sendErr)
-				continue
-			}
-		}
-		from, err := message.GetSender(false)
-		if err != nil {
-			sendErr := &SendError{Reason: ErrGetSender, errlist: []error{err}, isTemp: isTempError(err)}
-			message.sendError = sendErr
-			errs = append(errs, sendErr)
-			continue
-		}
-		rcpts, err := message.GetRecipients()
-		if err != nil {
-			sendErr := &SendError{Reason: ErrGetRcpts, errlist: []error{err}, isTemp: isTempError(err)}
-			message.sendError = sendErr
-			errs = append(errs, sendErr)
-			continue
-		}
+	for id, message := range messages {
+		if sendErr := c.sendSingleMsg(message); sendErr != nil {
+			messages[id].sendError = sendErr
 
-		if c.dsn {
-			if c.dsnmrtype != "" {
-				c.smtpClient.SetDSNMailReturnOption(string(c.dsnmrtype))
+			var msgSendErr *SendError
+			if errors.As(sendErr, &msgSendErr) {
+				errs = append(errs, msgSendErr)
 			}
 		}
-		if err = c.smtpClient.Mail(from); err != nil {
-			sendErr := &SendError{Reason: ErrSMTPMailFrom, errlist: []error{err}, isTemp: isTempError(err)}
-			if resetSendErr := c.smtpClient.Reset(); resetSendErr != nil {
-				sendErr.errlist = append(sendErr.errlist, resetSendErr)
-			}
-			message.sendError = sendErr
-			errs = append(errs, sendErr)
-			continue
-		}
-		failed := false
-		rcptSendErr := &SendError{}
-		rcptSendErr.errlist = make([]error, 0)
-		rcptSendErr.rcpt = make([]string, 0)
-		rcptNotifyOpt := strings.Join(c.dsnrntype, ",")
-		c.smtpClient.SetDSNRcptNotifyOption(rcptNotifyOpt)
-		for _, rcpt := range rcpts {
-			if err = c.smtpClient.Rcpt(rcpt); err != nil {
-				rcptSendErr.Reason = ErrSMTPRcptTo
-				rcptSendErr.errlist = append(rcptSendErr.errlist, err)
-				rcptSendErr.rcpt = append(rcptSendErr.rcpt, rcpt)
-				rcptSendErr.isTemp = isTempError(err)
-				failed = true
-			}
-		}
-		if failed {
-			if resetSendErr := c.smtpClient.Reset(); resetSendErr != nil {
-				rcptSendErr.errlist = append(rcptSendErr.errlist, err)
-			}
-			message.sendError = rcptSendErr
-			errs = append(errs, rcptSendErr)
-			continue
-		}
-		writer, err := c.smtpClient.Data()
-		if err != nil {
-			sendErr := &SendError{Reason: ErrSMTPData, errlist: []error{err}, isTemp: isTempError(err)}
-			message.sendError = sendErr
-			errs = append(errs, sendErr)
-			continue
-		}
-		_, err = message.WriteTo(writer)
-		if err != nil {
-			sendErr := &SendError{Reason: ErrWriteContent, errlist: []error{err}, isTemp: isTempError(err)}
-			message.sendError = sendErr
-			errs = append(errs, sendErr)
-			continue
-		}
-		message.isDelivered = true
-
-		if err = writer.Close(); err != nil {
-			sendErr := &SendError{Reason: ErrSMTPDataClose, errlist: []error{err}, isTemp: isTempError(err)}
-			message.sendError = sendErr
-			errs = append(errs, sendErr)
-			continue
-		}
-
-		if err = c.Reset(); err != nil {
-			sendErr := &SendError{Reason: ErrSMTPReset, errlist: []error{err}, isTemp: isTempError(err)}
-			message.sendError = sendErr
-			errs = append(errs, sendErr)
-			continue
-		}
-		if err = c.checkConn(); err != nil {
-			sendErr := &SendError{Reason: ErrConnCheck, errlist: []error{err}, isTemp: isTempError(err)}
-			message.sendError = sendErr
-			errs = append(errs, sendErr)
-			continue
-		}
 	}
 
 	if len(errs) > 0 {

client_120.go

@@ -9,7 +9,6 @@ package mail
 
 import (
 	"errors"
-	"strings"
 )
 
 // Send sends out the mail message
@@ -18,92 +17,16 @@ func (c *Client) Send(messages ...*Msg) (returnErr error) {
 		returnErr = &SendError{Reason: ErrConnCheck, errlist: []error{err}, isTemp: isTempError(err)}
 		return
 	}
-	for _, message := range messages {
-		message.sendError = nil
-		if message.encoding == NoEncoding {
-			if ok, _ := c.smtpClient.Extension("8BITMIME"); !ok {
-				message.sendError = &SendError{Reason: ErrNoUnencoded, isTemp: false}
-				returnErr = errors.Join(returnErr, message.sendError)
-				continue
-			}
-		}
-		from, err := message.GetSender(false)
-		if err != nil {
-			message.sendError = &SendError{Reason: ErrGetSender, errlist: []error{err}, isTemp: isTempError(err)}
-			returnErr = errors.Join(returnErr, message.sendError)
-			continue
-		}
-		rcpts, err := message.GetRecipients()
-		if err != nil {
-			message.sendError = &SendError{Reason: ErrGetRcpts, errlist: []error{err}, isTemp: isTempError(err)}
-			returnErr = errors.Join(returnErr, message.sendError)
-			continue
-		}
 
-		if c.dsn {
-			if c.dsnmrtype != "" {
-				c.smtpClient.SetDSNMailReturnOption(string(c.dsnmrtype))
-			}
-		}
-		if err = c.smtpClient.Mail(from); err != nil {
-			message.sendError = &SendError{Reason: ErrSMTPMailFrom, errlist: []error{err}, isTemp: isTempError(err)}
-			returnErr = errors.Join(returnErr, message.sendError)
-			if resetSendErr := c.smtpClient.Reset(); resetSendErr != nil {
-				returnErr = errors.Join(returnErr, resetSendErr)
-			}
-			continue
-		}
-		failed := false
-		rcptSendErr := &SendError{}
-		rcptSendErr.errlist = make([]error, 0)
-		rcptSendErr.rcpt = make([]string, 0)
-		rcptNotifyOpt := strings.Join(c.dsnrntype, ",")
-		c.smtpClient.SetDSNRcptNotifyOption(rcptNotifyOpt)
-		for _, rcpt := range rcpts {
-			if err = c.smtpClient.Rcpt(rcpt); err != nil {
-				rcptSendErr.Reason = ErrSMTPRcptTo
-				rcptSendErr.errlist = append(rcptSendErr.errlist, err)
-				rcptSendErr.rcpt = append(rcptSendErr.rcpt, rcpt)
-				rcptSendErr.isTemp = isTempError(err)
-				failed = true
-			}
-		}
-		if failed {
-			if resetSendErr := c.smtpClient.Reset(); resetSendErr != nil {
-				returnErr = errors.Join(returnErr, resetSendErr)
-			}
-			message.sendError = rcptSendErr
-			returnErr = errors.Join(returnErr, message.sendError)
-			continue
-		}
-		writer, err := c.smtpClient.Data()
-		if err != nil {
-			message.sendError = &SendError{Reason: ErrSMTPData, errlist: []error{err}, isTemp: isTempError(err)}
-			returnErr = errors.Join(returnErr, message.sendError)
-			continue
-		}
-		_, err = message.WriteTo(writer)
-		if err != nil {
-			message.sendError = &SendError{Reason: ErrWriteContent, errlist: []error{err}, isTemp: isTempError(err)}
-			returnErr = errors.Join(returnErr, message.sendError)
-			continue
-		}
-		message.isDelivered = true
+	var errs []error
+	defer func() {
+		returnErr = errors.Join(errs...)
+	}()
 
-		if err = writer.Close(); err != nil {
-			message.sendError = &SendError{Reason: ErrSMTPDataClose, errlist: []error{err}, isTemp: isTempError(err)}
-			returnErr = errors.Join(returnErr, message.sendError)
-			continue
-		}
-
-		if err = c.Reset(); err != nil {
-			message.sendError = &SendError{Reason: ErrSMTPReset, errlist: []error{err}, isTemp: isTempError(err)}
-			returnErr = errors.Join(returnErr, message.sendError)
-			continue
-		}
-		if err = c.checkConn(); err != nil {
-			message.sendError = &SendError{Reason: ErrConnCheck, errlist: []error{err}, isTemp: isTempError(err)}
-			returnErr = errors.Join(returnErr, message.sendError)
+	for id, message := range messages {
+		if sendErr := c.sendSingleMsg(message); sendErr != nil {
+			messages[id].sendError = sendErr
+			errs = append(errs, sendErr)
 		}
 	}
 

client_test.go

@@ -5,6 +5,7 @@
 package mail
 
 import (
+	"bufio"
 	"context"
 	"crypto/tls"
 	"errors"
@@ -21,11 +22,18 @@ import (
 	"github.com/wneessen/go-mail/smtp"
 )
 
+const (
 	// DefaultHost is used as default hostname for the Client
-const DefaultHost = "localhost"
-
-// TestRcpt
-const TestRcpt = "go-mail@mytrashmailer.com"
+	DefaultHost = "localhost"
+	// TestRcpt is a trash mail address to send test mails to
+	TestRcpt = "go-mail@mytrashmailer.com"
+	// TestServerProto is the protocol used for the simple SMTP test server
+	TestServerProto = "tcp"
+	// TestServerAddr is the address the simple SMTP test server listens on
+	TestServerAddr = "127.0.0.1"
+	// TestServerPortBase is the base port for the simple SMTP test server
+	TestServerPortBase = 2025
+)
 
 // TestNewClient tests the NewClient() method with its default options
 func TestNewClient(t *testing.T) {
@@ -629,7 +637,7 @@ func TestClient_DialWithContext(t *testing.T) {
 // TestClient_DialWithContext_Fallback tests the Client.DialWithContext method with the fallback
 // port functionality
 func TestClient_DialWithContext_Fallback(t *testing.T) {
-	c, err := getTestConnection(true)
+	c, err := getTestConnectionNoTestPort(true)
 	if err != nil {
 		t.Skipf("failed to create test client: %s. Skipping tests", err)
 	}
@@ -1251,6 +1259,475 @@ func TestClient_DialAndSendWithContext_withSendError(t *testing.T) {
 	}
 }
 
+func TestClient_SendErrorNoEncoding(t *testing.T) {
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	featureSet := "250-AUTH PLAIN\r\n250-DSN\r\n250 SMTPUTF8"
+	serverPort := TestServerPortBase + 1
+	go func() {
+		if err := simpleSMTPServer(ctx, featureSet, false, serverPort); err != nil {
+			t.Errorf("failed to start test server: %s", err)
+			return
+		}
+	}()
+	time.Sleep(time.Millisecond * 300)
+
+	message := NewMsg()
+	if err := message.From("valid-from@domain.tld"); err != nil {
+		t.Errorf("failed to set FROM address: %s", err)
+		return
+	}
+	if err := message.To("valid-to@domain.tld"); err != nil {
+		t.Errorf("failed to set TO address: %s", err)
+		return
+	}
+	message.Subject("Test subject")
+	message.SetBodyString(TypeTextPlain, "Test body")
+	message.SetMessageIDWithValue("this.is.a.message.id")
+	message.SetEncoding(NoEncoding)
+
+	client, err := NewClient(TestServerAddr, WithPort(serverPort),
+		WithTLSPortPolicy(NoTLS), WithSMTPAuth(SMTPAuthPlain),
+		WithUsername("toni@tester.com"),
+		WithPassword("V3ryS3cr3t+"))
+	if err != nil {
+		t.Errorf("unable to create new client: %s", err)
+	}
+	if err = client.DialWithContext(context.Background()); err != nil {
+		t.Errorf("failed to dial to test server: %s", err)
+	}
+	if err = client.Send(message); err == nil {
+		t.Error("expected Send() to fail but didn't")
+	}
+
+	var sendErr *SendError
+	if !errors.As(err, &sendErr) {
+		t.Errorf("expected *SendError type as returned error, but got %T", sendErr)
+	}
+	if errors.As(err, &sendErr) {
+		if sendErr.IsTemp() {
+			t.Errorf("expected permanent error but IsTemp() returned true")
+		}
+		if sendErr.Reason != ErrNoUnencoded {
+			t.Errorf("expected ErrNoUnencoded error, but got %s", sendErr.Reason)
+		}
+		if !strings.EqualFold(sendErr.MessageID(), "<this.is.a.message.id>") {
+			t.Errorf("expected message ID: %q, but got %q", "<this.is.a.message.id>",
+				sendErr.MessageID())
+		}
+		if sendErr.Msg() == nil {
+			t.Errorf("expected message to be set, but got nil")
+		}
+	}
+
+	if err = client.Close(); err != nil {
+		t.Errorf("failed to close server connection: %s", err)
+	}
+}
+
+func TestClient_SendErrorMailFrom(t *testing.T) {
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	serverPort := TestServerPortBase + 2
+	featureSet := "250-AUTH PLAIN\r\n250-8BITMIME\r\n250-DSN\r\n250 SMTPUTF8"
+	go func() {
+		if err := simpleSMTPServer(ctx, featureSet, false, serverPort); err != nil {
+			t.Errorf("failed to start test server: %s", err)
+			return
+		}
+	}()
+	time.Sleep(time.Millisecond * 300)
+
+	message := NewMsg()
+	if err := message.From("invalid-from@domain.tld"); err != nil {
+		t.Errorf("failed to set FROM address: %s", err)
+		return
+	}
+	if err := message.To("valid-to@domain.tld"); err != nil {
+		t.Errorf("failed to set TO address: %s", err)
+		return
+	}
+	message.Subject("Test subject")
+	message.SetBodyString(TypeTextPlain, "Test body")
+	message.SetMessageIDWithValue("this.is.a.message.id")
+
+	client, err := NewClient(TestServerAddr, WithPort(serverPort),
+		WithTLSPortPolicy(NoTLS), WithSMTPAuth(SMTPAuthPlain),
+		WithUsername("toni@tester.com"),
+		WithPassword("V3ryS3cr3t+"))
+	if err != nil {
+		t.Errorf("unable to create new client: %s", err)
+	}
+	if err = client.DialWithContext(context.Background()); err != nil {
+		t.Errorf("failed to dial to test server: %s", err)
+	}
+	if err = client.Send(message); err == nil {
+		t.Error("expected Send() to fail but didn't")
+	}
+
+	var sendErr *SendError
+	if !errors.As(err, &sendErr) {
+		t.Errorf("expected *SendError type as returned error, but got %T", sendErr)
+	}
+	if errors.As(err, &sendErr) {
+		if sendErr.IsTemp() {
+			t.Errorf("expected permanent error but IsTemp() returned true")
+		}
+		if sendErr.Reason != ErrSMTPMailFrom {
+			t.Errorf("expected ErrSMTPMailFrom error, but got %s", sendErr.Reason)
+		}
+		if !strings.EqualFold(sendErr.MessageID(), "<this.is.a.message.id>") {
+			t.Errorf("expected message ID: %q, but got %q", "<this.is.a.message.id>",
+				sendErr.MessageID())
+		}
+		if sendErr.Msg() == nil {
+			t.Errorf("expected message to be set, but got nil")
+		}
+	}
+
+	if err = client.Close(); err != nil {
+		t.Errorf("failed to close server connection: %s", err)
+	}
+}
+
+func TestClient_SendErrorMailFromReset(t *testing.T) {
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	serverPort := TestServerPortBase + 3
+	featureSet := "250-AUTH PLAIN\r\n250-8BITMIME\r\n250-DSN\r\n250 SMTPUTF8"
+	go func() {
+		if err := simpleSMTPServer(ctx, featureSet, true, serverPort); err != nil {
+			t.Errorf("failed to start test server: %s", err)
+			return
+		}
+	}()
+	time.Sleep(time.Millisecond * 300)
+
+	message := NewMsg()
+	if err := message.From("invalid-from@domain.tld"); err != nil {
+		t.Errorf("failed to set FROM address: %s", err)
+		return
+	}
+	if err := message.To("valid-to@domain.tld"); err != nil {
+		t.Errorf("failed to set TO address: %s", err)
+		return
+	}
+	message.Subject("Test subject")
+	message.SetBodyString(TypeTextPlain, "Test body")
+	message.SetMessageIDWithValue("this.is.a.message.id")
+
+	client, err := NewClient(TestServerAddr, WithPort(serverPort),
+		WithTLSPortPolicy(NoTLS), WithSMTPAuth(SMTPAuthPlain),
+		WithUsername("toni@tester.com"),
+		WithPassword("V3ryS3cr3t+"))
+	if err != nil {
+		t.Errorf("unable to create new client: %s", err)
+	}
+	if err = client.DialWithContext(context.Background()); err != nil {
+		t.Errorf("failed to dial to test server: %s", err)
+	}
+	if err = client.Send(message); err == nil {
+		t.Error("expected Send() to fail but didn't")
+	}
+
+	var sendErr *SendError
+	if !errors.As(err, &sendErr) {
+		t.Errorf("expected *SendError type as returned error, but got %T", sendErr)
+	}
+	if errors.As(err, &sendErr) {
+		if sendErr.IsTemp() {
+			t.Errorf("expected permanent error but IsTemp() returned true")
+		}
+		if sendErr.Reason != ErrSMTPMailFrom {
+			t.Errorf("expected ErrSMTPMailFrom error, but got %s", sendErr.Reason)
+		}
+		if !strings.EqualFold(sendErr.MessageID(), "<this.is.a.message.id>") {
+			t.Errorf("expected message ID: %q, but got %q", "<this.is.a.message.id>",
+				sendErr.MessageID())
+		}
+		if len(sendErr.errlist) != 2 {
+			t.Errorf("expected 2 errors, but got %d", len(sendErr.errlist))
+			return
+		}
+		if !strings.EqualFold(sendErr.errlist[0].Error(), "503 5.1.2 Invalid from: <invalid-from@domain.tld>") {
+			t.Errorf("expected error: %q, but got %q",
+				"503 5.1.2 Invalid from: <invalid-from@domain.tld>", sendErr.errlist[0].Error())
+		}
+		if !strings.EqualFold(sendErr.errlist[1].Error(), "500 5.1.2 Error: reset failed") {
+			t.Errorf("expected error: %q, but got %q",
+				"500 5.1.2 Error: reset failed", sendErr.errlist[1].Error())
+		}
+	}
+
+	if err = client.Close(); err != nil {
+		t.Errorf("failed to close server connection: %s", err)
+	}
+}
+
+func TestClient_SendErrorToReset(t *testing.T) {
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	serverPort := TestServerPortBase + 4
+	featureSet := "250-AUTH PLAIN\r\n250-8BITMIME\r\n250-DSN\r\n250 SMTPUTF8"
+	go func() {
+		if err := simpleSMTPServer(ctx, featureSet, true, serverPort); err != nil {
+			t.Errorf("failed to start test server: %s", err)
+			return
+		}
+	}()
+	time.Sleep(time.Millisecond * 300)
+
+	message := NewMsg()
+	if err := message.From("valid-from@domain.tld"); err != nil {
+		t.Errorf("failed to set FROM address: %s", err)
+		return
+	}
+	if err := message.To("invalid-to@domain.tld"); err != nil {
+		t.Errorf("failed to set TO address: %s", err)
+		return
+	}
+	message.Subject("Test subject")
+	message.SetBodyString(TypeTextPlain, "Test body")
+	message.SetMessageIDWithValue("this.is.a.message.id")
+
+	client, err := NewClient(TestServerAddr, WithPort(serverPort),
+		WithTLSPortPolicy(NoTLS), WithSMTPAuth(SMTPAuthPlain),
+		WithUsername("toni@tester.com"),
+		WithPassword("V3ryS3cr3t+"))
+	if err != nil {
+		t.Errorf("unable to create new client: %s", err)
+	}
+	if err = client.DialWithContext(context.Background()); err != nil {
+		t.Errorf("failed to dial to test server: %s", err)
+	}
+	if err = client.Send(message); err == nil {
+		t.Error("expected Send() to fail but didn't")
+	}
+
+	var sendErr *SendError
+	if !errors.As(err, &sendErr) {
+		t.Errorf("expected *SendError type as returned error, but got %T", sendErr)
+	}
+	if errors.As(err, &sendErr) {
+		if sendErr.IsTemp() {
+			t.Errorf("expected permanent error but IsTemp() returned true")
+		}
+		if sendErr.Reason != ErrSMTPRcptTo {
+			t.Errorf("expected ErrSMTPRcptTo error, but got %s", sendErr.Reason)
+		}
+		if !strings.EqualFold(sendErr.MessageID(), "<this.is.a.message.id>") {
+			t.Errorf("expected message ID: %q, but got %q", "<this.is.a.message.id>",
+				sendErr.MessageID())
+		}
+		if len(sendErr.errlist) != 2 {
+			t.Errorf("expected 2 errors, but got %d", len(sendErr.errlist))
+			return
+		}
+		if !strings.EqualFold(sendErr.errlist[0].Error(), "500 5.1.2 Invalid to: <invalid-to@domain.tld>") {
+			t.Errorf("expected error: %q, but got %q",
+				"500 5.1.2 Invalid to: <invalid-to@domain.tld>", sendErr.errlist[0].Error())
+		}
+		if !strings.EqualFold(sendErr.errlist[1].Error(), "500 5.1.2 Error: reset failed") {
+			t.Errorf("expected error: %q, but got %q",
+				"500 5.1.2 Error: reset failed", sendErr.errlist[1].Error())
+		}
+	}
+
+	if err = client.Close(); err != nil {
+		t.Errorf("failed to close server connection: %s", err)
+	}
+}
+
+func TestClient_SendErrorDataClose(t *testing.T) {
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	serverPort := TestServerPortBase + 5
+	featureSet := "250-AUTH PLAIN\r\n250-8BITMIME\r\n250-DSN\r\n250 SMTPUTF8"
+	go func() {
+		if err := simpleSMTPServer(ctx, featureSet, false, serverPort); err != nil {
+			t.Errorf("failed to start test server: %s", err)
+			return
+		}
+	}()
+	time.Sleep(time.Millisecond * 300)
+
+	message := NewMsg()
+	if err := message.From("valid-from@domain.tld"); err != nil {
+		t.Errorf("failed to set FROM address: %s", err)
+		return
+	}
+	if err := message.To("valid-to@domain.tld"); err != nil {
+		t.Errorf("failed to set TO address: %s", err)
+		return
+	}
+	message.Subject("Test subject")
+	message.SetBodyString(TypeTextPlain, "DATA close should fail")
+	message.SetMessageIDWithValue("this.is.a.message.id")
+
+	client, err := NewClient(TestServerAddr, WithPort(serverPort),
+		WithTLSPortPolicy(NoTLS), WithSMTPAuth(SMTPAuthPlain),
+		WithUsername("toni@tester.com"),
+		WithPassword("V3ryS3cr3t+"))
+	if err != nil {
+		t.Errorf("unable to create new client: %s", err)
+	}
+	if err = client.DialWithContext(context.Background()); err != nil {
+		t.Errorf("failed to dial to test server: %s", err)
+	}
+	if err = client.Send(message); err == nil {
+		t.Error("expected Send() to fail but didn't")
+	}
+
+	var sendErr *SendError
+	if !errors.As(err, &sendErr) {
+		t.Errorf("expected *SendError type as returned error, but got %T", sendErr)
+	}
+	if errors.As(err, &sendErr) {
+		if sendErr.IsTemp() {
+			t.Errorf("expected permanent error but IsTemp() returned true")
+		}
+		if sendErr.Reason != ErrSMTPDataClose {
+			t.Errorf("expected ErrSMTPDataClose error, but got %s", sendErr.Reason)
+		}
+		if !strings.EqualFold(sendErr.MessageID(), "<this.is.a.message.id>") {
+			t.Errorf("expected message ID: %q, but got %q", "<this.is.a.message.id>",
+				sendErr.MessageID())
+		}
+	}
+
+	if err = client.Close(); err != nil {
+		t.Errorf("failed to close server connection: %s", err)
+	}
+}
+
+func TestClient_SendErrorDataWrite(t *testing.T) {
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	serverPort := TestServerPortBase + 6
+	featureSet := "250-AUTH PLAIN\r\n250-8BITMIME\r\n250-DSN\r\n250 SMTPUTF8"
+	go func() {
+		if err := simpleSMTPServer(ctx, featureSet, false, serverPort); err != nil {
+			t.Errorf("failed to start test server: %s", err)
+			return
+		}
+	}()
+	time.Sleep(time.Millisecond * 300)
+
+	message := NewMsg()
+	if err := message.From("valid-from@domain.tld"); err != nil {
+		t.Errorf("failed to set FROM address: %s", err)
+		return
+	}
+	if err := message.To("valid-to@domain.tld"); err != nil {
+		t.Errorf("failed to set TO address: %s", err)
+		return
+	}
+	message.Subject("Test subject")
+	message.SetBodyString(TypeTextPlain, "DATA write should fail")
+	message.SetMessageIDWithValue("this.is.a.message.id")
+	message.SetGenHeader("X-Test-Header", "DATA write should fail")
+
+	client, err := NewClient(TestServerAddr, WithPort(serverPort),
+		WithTLSPortPolicy(NoTLS), WithSMTPAuth(SMTPAuthPlain),
+		WithUsername("toni@tester.com"),
+		WithPassword("V3ryS3cr3t+"))
+	if err != nil {
+		t.Errorf("unable to create new client: %s", err)
+	}
+	if err = client.DialWithContext(context.Background()); err != nil {
+		t.Errorf("failed to dial to test server: %s", err)
+	}
+	if err = client.Send(message); err == nil {
+		t.Error("expected Send() to fail but didn't")
+	}
+
+	var sendErr *SendError
+	if !errors.As(err, &sendErr) {
+		t.Errorf("expected *SendError type as returned error, but got %T", sendErr)
+	}
+	if errors.As(err, &sendErr) {
+		if sendErr.IsTemp() {
+			t.Errorf("expected permanent error but IsTemp() returned true")
+		}
+		if sendErr.Reason != ErrSMTPDataClose {
+			t.Errorf("expected ErrSMTPDataClose error, but got %s", sendErr.Reason)
+		}
+		if !strings.EqualFold(sendErr.MessageID(), "<this.is.a.message.id>") {
+			t.Errorf("expected message ID: %q, but got %q", "<this.is.a.message.id>",
+				sendErr.MessageID())
+		}
+	}
+}
+
+func TestClient_SendErrorReset(t *testing.T) {
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	serverPort := TestServerPortBase + 7
+	featureSet := "250-AUTH PLAIN\r\n250-8BITMIME\r\n250-DSN\r\n250 SMTPUTF8"
+	go func() {
+		if err := simpleSMTPServer(ctx, featureSet, true, serverPort); err != nil {
+			t.Errorf("failed to start test server: %s", err)
+			return
+		}
+	}()
+	time.Sleep(time.Millisecond * 300)
+
+	message := NewMsg()
+	if err := message.From("valid-from@domain.tld"); err != nil {
+		t.Errorf("failed to set FROM address: %s", err)
+		return
+	}
+	if err := message.To("valid-to@domain.tld"); err != nil {
+		t.Errorf("failed to set TO address: %s", err)
+		return
+	}
+	message.Subject("Test subject")
+	message.SetBodyString(TypeTextPlain, "Test body")
+	message.SetMessageIDWithValue("this.is.a.message.id")
+
+	client, err := NewClient(TestServerAddr, WithPort(serverPort),
+		WithTLSPortPolicy(NoTLS), WithSMTPAuth(SMTPAuthPlain),
+		WithUsername("toni@tester.com"),
+		WithPassword("V3ryS3cr3t+"))
+	if err != nil {
+		t.Errorf("unable to create new client: %s", err)
+	}
+	if err = client.DialWithContext(context.Background()); err != nil {
+		t.Errorf("failed to dial to test server: %s", err)
+	}
+	if err = client.Send(message); err == nil {
+		t.Error("expected Send() to fail but didn't")
+	}
+
+	var sendErr *SendError
+	if !errors.As(err, &sendErr) {
+		t.Errorf("expected *SendError type as returned error, but got %T", sendErr)
+	}
+	if errors.As(err, &sendErr) {
+		if sendErr.IsTemp() {
+			t.Errorf("expected permanent error but IsTemp() returned true")
+		}
+		if sendErr.Reason != ErrSMTPReset {
+			t.Errorf("expected ErrSMTPReset error, but got %s", sendErr.Reason)
+		}
+		if !strings.EqualFold(sendErr.MessageID(), "<this.is.a.message.id>") {
+			t.Errorf("expected message ID: %q, but got %q", "<this.is.a.message.id>",
+				sendErr.MessageID())
+		}
+	}
+
+	if err = client.Close(); err != nil {
+		t.Errorf("failed to close server connection: %s", err)
+	}
+}
+
 // getTestConnection takes environment variables to establish a connection to a real
 // SMTP server to test all functionality that requires a connection
 func getTestConnection(auth bool) (*Client, error) {
@@ -1302,6 +1779,50 @@ func getTestConnection(auth bool) (*Client, error) {
 	return c, nil
 }
 
+// getTestConnectionNoTestPort takes environment variables (except the port) to establish a
+// connection to a real SMTP server to test all functionality that requires a connection
+func getTestConnectionNoTestPort(auth bool) (*Client, error) {
+	if os.Getenv("TEST_SKIP_ONLINE") != "" {
+		return nil, fmt.Errorf("env variable TEST_SKIP_ONLINE is set. Skipping online tests")
+	}
+	th := os.Getenv("TEST_HOST")
+	if th == "" {
+		return nil, fmt.Errorf("no TEST_HOST set")
+	}
+	sv := false
+	if sve := os.Getenv("TEST_TLS_SKIP_VERIFY"); sve != "" {
+		sv = true
+	}
+	c, err := NewClient(th)
+	if err != nil {
+		return c, err
+	}
+	c.tlsconfig.InsecureSkipVerify = sv
+	if auth {
+		st := os.Getenv("TEST_SMTPAUTH_TYPE")
+		if st != "" {
+			c.SetSMTPAuth(SMTPAuthType(st))
+		}
+		u := os.Getenv("TEST_SMTPAUTH_USER")
+		if u != "" {
+			c.SetUsername(u)
+		}
+		p := os.Getenv("TEST_SMTPAUTH_PASS")
+		if p != "" {
+			c.SetPassword(p)
+		}
+		// We don't want to log authentication data in tests
+		c.SetDebugLog(false)
+	}
+	if err := c.DialWithContext(context.Background()); err != nil {
+		return c, fmt.Errorf("connection to test server failed: %w", err)
+	}
+	if err := c.Close(); err != nil {
+		return c, fmt.Errorf("disconnect from test server failed: %w", err)
+	}
+	return c, nil
+}
+
 // getTestClient takes environment variables to establish a client without connecting
 // to the SMTP server
 func getTestClient(auth bool) (*Client, error) {
@@ -1357,7 +1878,14 @@ func getTestConnectionWithDSN(auth bool) (*Client, error) {
 	if th == "" {
 		return nil, fmt.Errorf("no TEST_HOST set")
 	}
-	c, err := NewClient(th, WithDSN())
+	tp := 25
+	if tps := os.Getenv("TEST_PORT"); tps != "" {
+		tpi, err := strconv.Atoi(tps)
+		if err == nil {
+			tp = tpi
+		}
+	}
+	c, err := NewClient(th, WithDSN(), WithPort(tp))
 	if err != nil {
 		return c, err
 	}
@@ -1494,3 +2022,155 @@ func (f faker) RemoteAddr() net.Addr             { return nil }
 func (f faker) SetDeadline(time.Time) error      { return nil }
 func (f faker) SetReadDeadline(time.Time) error  { return nil }
 func (f faker) SetWriteDeadline(time.Time) error { return nil }
+
+// simpleSMTPServer starts a simple TCP server that resonds to SMTP commands.
+// The provided featureSet represents in what the server responds to EHLO command
+// failReset controls if a RSET succeeds
+func simpleSMTPServer(ctx context.Context, featureSet string, failReset bool, port int) error {
+	listener, err := net.Listen(TestServerProto, fmt.Sprintf("%s:%d", TestServerAddr, port))
+	if err != nil {
+		return fmt.Errorf("unable to listen on %s://%s: %w", TestServerProto, TestServerAddr, err)
+	}
+
+	defer func() {
+		if err := listener.Close(); err != nil {
+			fmt.Printf("unable to close listener: %s\n", err)
+			os.Exit(1)
+		}
+	}()
+
+	for {
+		select {
+		case <-ctx.Done():
+			return nil
+		default:
+			connection, err := listener.Accept()
+			var opErr *net.OpError
+			if err != nil {
+				if errors.As(err, &opErr) && opErr.Temporary() {
+					continue
+				}
+				return fmt.Errorf("unable to accept connection: %w", err)
+			}
+			handleTestServerConnection(connection, featureSet, failReset)
+		}
+	}
+}
+
+func handleTestServerConnection(connection net.Conn, featureSet string, failReset bool) {
+	defer func() {
+		if err := connection.Close(); err != nil {
+			fmt.Printf("unable to close connection: %s\n", err)
+		}
+	}()
+
+	reader := bufio.NewReader(connection)
+	writer := bufio.NewWriter(connection)
+
+	writeLine := func(data string) error {
+		_, err := writer.WriteString(data + "\r\n")
+		if err != nil {
+			return fmt.Errorf("unable to write line: %w", err)
+		}
+		return writer.Flush()
+	}
+	writeOK := func() {
+		_ = writeLine("250 2.0.0 OK")
+	}
+
+	if err := writeLine("220 go-mail test server ready ESMTP"); err != nil {
+		fmt.Printf("unable to write to client: %s\n", err)
+		return
+	}
+
+	data, err := reader.ReadString('\n')
+	if err != nil {
+		fmt.Printf("unable to read from connection: %s\n", err)
+		return
+	}
+	if !strings.HasPrefix(data, "EHLO") && !strings.HasPrefix(data, "HELO") {
+		fmt.Printf("expected EHLO, got %q", data)
+		return
+	}
+	if err = writeLine("250-localhost.localdomain\r\n" + featureSet); err != nil {
+		fmt.Printf("unable to write to connection: %s\n", err)
+		return
+	}
+
+	for {
+		data, err = reader.ReadString('\n')
+		if err != nil {
+			if errors.Is(err, io.EOF) {
+				break
+			}
+			fmt.Println("Error reading data:", err)
+			break
+		}
+
+		var datastring string
+		data = strings.TrimSpace(data)
+		switch {
+		case strings.HasPrefix(data, "MAIL FROM:"):
+			from := strings.TrimPrefix(data, "MAIL FROM:")
+			from = strings.ReplaceAll(from, "BODY=8BITMIME", "")
+			from = strings.ReplaceAll(from, "SMTPUTF8", "")
+			from = strings.TrimSpace(from)
+			if !strings.EqualFold(from, "<valid-from@domain.tld>") {
+				_ = writeLine(fmt.Sprintf("503 5.1.2 Invalid from: %s", from))
+				break
+			}
+			writeOK()
+		case strings.HasPrefix(data, "RCPT TO:"):
+			to := strings.TrimPrefix(data, "RCPT TO:")
+			to = strings.TrimSpace(to)
+			if !strings.EqualFold(to, "<valid-to@domain.tld>") {
+				_ = writeLine(fmt.Sprintf("500 5.1.2 Invalid to: %s", to))
+				break
+			}
+			writeOK()
+		case strings.HasPrefix(data, "AUTH PLAIN"):
+			auth := strings.TrimPrefix(data, "AUTH PLAIN ")
+			if !strings.EqualFold(auth, "AHRvbmlAdGVzdGVyLmNvbQBWM3J5UzNjcjN0Kw==") {
+				_ = writeLine("535 5.7.8 Error: authentication failed")
+				break
+			}
+			_ = writeLine("235 2.7.0 Authentication successful")
+		case strings.EqualFold(data, "DATA"):
+			_ = writeLine("354 End data with <CR><LF>.<CR><LF>")
+			for {
+				ddata, derr := reader.ReadString('\n')
+				if derr != nil {
+					fmt.Printf("failed to read DATA data from connection: %s\n", derr)
+					break
+				}
+				ddata = strings.TrimSpace(ddata)
+				if strings.EqualFold(ddata, "DATA write should fail") {
+					_ = writeLine("500 5.0.0 Error during DATA transmission")
+					break
+				}
+				if ddata == "." {
+					if strings.Contains(datastring, "DATA close should fail") {
+						_ = writeLine("500 5.0.0 Error during DATA closing")
+						break
+					}
+					_ = writeLine("250 2.0.0 Ok: queued as 1234567890")
+					break
+				}
+				datastring += ddata + "\n"
+			}
+		case strings.EqualFold(data, "noop"),
+			strings.EqualFold(data, "vrfy"):
+			writeOK()
+		case strings.EqualFold(data, "rset"):
+			if failReset {
+				_ = writeLine("500 5.1.2 Error: reset failed")
+				break
+			}
+			writeOK()
+		case strings.EqualFold(data, "quit"):
+			_ = writeLine("221 2.0.0 Bye")
+		default:
+			_ = writeLine("500 5.5.2 Error: bad syntax")
+		}
+	}
+}

connpool.go

@@ -0,0 +1,89 @@
+// SPDX-FileCopyrightText: 2022-2024 The go-mail Authors
+//
+// SPDX-License-Identifier: MIT
+
+package mail
+
+import (
+	"errors"
+	"net"
+	"sync"
+)
+
+// Parts of the connection pool code is forked from https://github.com/fatih/pool/
+// Thanks to Fatih Arslan and the project contributors for providing this great
+// concurrency template.
+
+var (
+	ErrPoolInvalidCap = errors.New("invalid connection pool capacity settings")
+)
+
+// Pool interface describes a connection pool implementation. A Pool is
+// thread-/go-routine safe.
+type Pool interface {
+	// Get returns a new connection from the pool. Closing the connections returns
+	// it back into the Pool. Closing a connection when the Pool is destroyed or
+	// full will be counted as an error.
+	Get() (net.Conn, error)
+
+	// Close closes the pool and all its connections. After Close() the pool is
+	// no longer usable.
+	Close()
+
+	// Len returns the current number of connections of the pool.
+	Len() int
+}
+
+// connPool implements the Pool interface
+type connPool struct {
+	// mutex is used to synchronize access to the connection pool to ensure thread-safe operations
+	mutex sync.RWMutex
+	// conns is a channel used to manage and distribute net.Conn objects within the connection pool
+	conns chan net.Conn
+	// dialCtx represents the actual net.Conn returned by the DialContextFunc
+	dialCtx DialContextFunc
+}
+
+// NewConnPool returns a new pool based on buffered channels with an initial
+// capacity and maximum capacity. The DialContextFunc is used when the initial
+// capacity is greater than zero to fill the pool. A zero initialCap doesn't
+// fill the Pool until a new Get() is called. During a Get(), if there is no
+// new connection available in the pool, a new connection will be created via
+// the corresponding DialContextFunc() method.
+func NewConnPool(initialCap, maxCap int, dialCtxFunc DialContextFunc) (Pool, error) {
+	if initialCap < 0 || maxCap <= 0 || initialCap > maxCap {
+		return nil, ErrPoolInvalidCap
+	}
+
+	pool := &connPool{
+		conns:   make(chan net.Conn, maxCap),
+		dialCtx: dialCtxFunc,
+	}
+
+	// create initial connections, if something goes wrong,
+	// just close the pool error out.
+	for i := 0; i < initialCap; i++ {
+		/*
+			conn, err := dialCtxFunc()
+			if err != nil {
+				pool.Close()
+				return nil, fmt.Errorf("factory is not able to fill the pool: %s", err)
+			}
+			c.conns <- conn
+
+		*/
+	}
+
+	return pool, nil
+}
+
+func (c *connPool) Get() (net.Conn, error) {
+	return nil, nil
+}
+func (c *connPool) Close() {
+	return
+}
+
+func (c *connPool) Len() int {
+	return 0
+}

msg.go

@@ -467,8 +467,8 @@ func (m *Msg) SetMessageID() {
 	if err != nil {
 		hostname = "localhost.localdomain"
 	}
-	randNumPrimary, _ := randNum(100000000)
-	randNumSecondary, _ := randNum(10000)
+	randNumPrimary := randNum(100000000)
+	randNumSecondary := randNum(10000)
 	randString, _ := randomStringSecure(17)
 	procID := os.Getpid() * randNumSecondary
 	messageID := fmt.Sprintf("%d.%d%d.%s@%s", procID, randNumPrimary, randNumSecondary,
@@ -476,6 +476,17 @@ func (m *Msg) SetMessageID() {
 	m.SetMessageIDWithValue(messageID)
 }
 
+// GetMessageID returns the message ID of the Msg as string value. If no message ID
+// is set, an empty string will be returned
+func (m *Msg) GetMessageID() string {
+	if msgidheader, ok := m.genHeader[HeaderMessageID]; ok {
+		if len(msgidheader) > 0 {
+			return msgidheader[0]
+		}
+	}
+	return ""
+}
+
 // SetMessageIDWithValue sets the message id for the mail
 func (m *Msg) SetMessageIDWithValue(messageID string) {
 	m.SetGenHeader(HeaderMessageID, fmt.Sprintf("<%s>", messageID))

msg_nowin_test.go

@@ -61,3 +61,25 @@ func TestMsg_WriteToSendmail(t *testing.T) {
 		t.Errorf("WriteToSendmail failed: %s", err)
 	}
 }
+
+func TestMsg_WriteToTempFileFailed(t *testing.T) {
+	m := NewMsg()
+	_ = m.From("Toni Tester <tester@example.com>")
+	_ = m.To("Ellenor Tester <ellinor@example.com>")
+	m.SetBodyString(TypeTextPlain, "This is a test")
+
+	curTmpDir := os.Getenv("TMPDIR")
+	defer func() {
+		if err := os.Setenv("TMPDIR", curTmpDir); err != nil {
+			t.Errorf("failed to set TMPDIR environment variable: %s", err)
+		}
+	}()
+
+	if err := os.Setenv("TMPDIR", "/invalid/directory/that/does/not/exist"); err != nil {
+		t.Errorf("failed to set TMPDIR environment variable: %s", err)
+	}
+	_, err := m.WriteToTempFile()
+	if err == nil {
+		t.Errorf("WriteToTempFile() did not fail as expected")
+	}
+}

msg_test.go

@@ -786,13 +786,11 @@ func TestMsg_SetMessageIDWithValue(t *testing.T) {
 // TestMsg_SetMessageIDRandomness tests the randomness of Msg.SetMessageID methods
 func TestMsg_SetMessageIDRandomness(t *testing.T) {
 	var mids []string
-	for i := 0; i < 100; i++ {
+	for i := 0; i < 50_000; i++ {
 		m := NewMsg()
 		m.SetMessageID()
-		mid := m.GetGenHeader(HeaderMessageID)
-		if len(mid) > 0 {
-			mids = append(mids, mid[0])
-		}
+		mid := m.GetMessageID()
+		mids = append(mids, mid)
 	}
 	c := make(map[string]int)
 	for i := range mids {
@@ -805,6 +803,21 @@ func TestMsg_SetMessageIDRandomness(t *testing.T) {
 	}
 }
 
+func TestMsg_GetMessageID(t *testing.T) {
+	expected := "this.is.a.message.id"
+	msg := NewMsg()
+	msg.SetMessageIDWithValue(expected)
+	val := msg.GetMessageID()
+	if !strings.EqualFold(val, fmt.Sprintf("<%s>", expected)) {
+		t.Errorf("GetMessageID() failed. Expected: %s, got: %s", fmt.Sprintf("<%s>", expected), val)
+	}
+	msg.genHeader[HeaderMessageID] = nil
+	val = msg.GetMessageID()
+	if val != "" {
+		t.Errorf("GetMessageID() failed. Expected empty string, got: %s", val)
+	}
+}
+
 // TestMsg_FromFormat tests the FromFormat and EnvelopeFrom methods for the Msg object
 func TestMsg_FromFormat(t *testing.T) {
 	tests := []struct {

random.go

@@ -7,8 +7,6 @@ package mail
 import (
 	"crypto/rand"
 	"encoding/binary"
-	"fmt"
-	"math/big"
 	"strings"
 )
 
@@ -52,23 +50,3 @@ func randomStringSecure(length int) (string, error) {
 
 	return randString.String(), nil
 }
-
-// randNum returns a random number with a maximum value of length
-func randNum(length int) (int, error) {
-	if length <= 0 {
-		return 0, fmt.Errorf("provided number is <= 0: %d", length)
-	}
-	length64 := big.NewInt(int64(length))
-	if !length64.IsUint64() {
-		return 0, fmt.Errorf("big.NewInt() generation returned negative value: %d", length64)
-	}
-	randNum64, err := rand.Int(rand.Reader, length64)
-	if err != nil {
-		return 0, err
-	}
-	randomNum := int(randNum64.Int64())
-	if randomNum < 0 {
-		return 0, fmt.Errorf("generated random number does not fit as int64: %d", randNum64)
-	}
-	return randomNum, nil
-}

random_119.go

@@ -0,0 +1,22 @@
+// SPDX-FileCopyrightText: 2022-2023 The go-mail Authors
+//
+// SPDX-License-Identifier: MIT
+
+//go:build go1.19 && !go1.20
+// +build go1.19,!go1.20
+
+package mail
+
+import (
+	"math/rand"
+	"time"
+)
+
+// randNum returns a random number with a maximum value of length
+func randNum(maxval int) int {
+	if maxval <= 0 {
+		return 0
+	}
+	rand.Seed(time.Now().UnixNano())
+	return rand.Intn(maxval)
+}

random_121.go

@@ -0,0 +1,20 @@
+// SPDX-FileCopyrightText: 2022-2023 The go-mail Authors
+//
+// SPDX-License-Identifier: MIT
+
+//go:build go1.20 && !go1.22
+// +build go1.20,!go1.22
+
+package mail
+
+import (
+	"math/rand"
+)
+
+// randNum returns a random number with a maximum value of length
+func randNum(maxval int) int {
+	if maxval <= 0 {
+		return 0
+	}
+	return rand.Intn(maxval)
+}

random_122.go

@@ -0,0 +1,22 @@
+// SPDX-FileCopyrightText: 2022-2023 The go-mail Authors
+//
+// SPDX-License-Identifier: MIT
+
+//go:build go1.22
+// +build go1.22
+
+package mail
+
+import (
+	"math/rand/v2"
+)
+
+// randNum returns a random number with a maximum value of maxval.
+// go-mail compiled with Go 1.22+ will make use of the novel math/rand/v2 interface
+// Older versions of Go will use math/rand
+func randNum(maxval int) int {
+	if maxval <= 0 {
+		return 0
+	}
+	return rand.IntN(maxval)
+}

random_test.go

@@ -55,16 +55,17 @@ func TestRandomNum(t *testing.T) {
 
 	for _, tc := range tt {
 		t.Run(tc.testName, func(t *testing.T) {
-			rn, err := randNum(tc.max)
-			if err != nil {
-				t.Errorf("random number generation failed: %s", err)
-			}
-			if rn < 0 {
-				t.Errorf("random number generation failed: %d is smaller than zero", rn)
-			}
+			rn := randNum(tc.max)
 			if rn > tc.max {
 				t.Errorf("random number generation failed: %d is bigger than given value %d", rn, tc.max)
 			}
 		})
 	}
 }
+
+func TestRandomNumZero(t *testing.T) {
+	rn := randNum(0)
+	if rn != 0 {
+		t.Errorf("random number generation failed: %d is not zero", rn)
+	}
+}

senderror.go

@@ -56,10 +56,11 @@ const (
 
 // SendError is an error wrapper for delivery errors of the Msg
 type SendError struct {
-	Reason  SendErrReason
-	isTemp  bool
+	affectedMsg *Msg
 	errlist     []error
+	isTemp      bool
 	rcpt        []string
+	Reason      SendErrReason
 }
 
 // SendErrReason represents a comparable reason on why the delivery failed
@@ -92,6 +93,11 @@ func (e *SendError) Error() string {
 			}
 		}
 	}
+	if e.affectedMsg != nil && e.affectedMsg.GetMessageID() != "" {
+		errMessage.WriteString(", affected message ID: ")
+		errMessage.WriteString(e.affectedMsg.GetMessageID())
+	}
+
 	return errMessage.String()
 }
 
@@ -112,6 +118,23 @@ func (e *SendError) IsTemp() bool {
 	return e.isTemp
 }
 
+// MessageID returns the message ID of the affected Msg that caused the error
+// If no message ID was set for the Msg, an empty string will be returned
+func (e *SendError) MessageID() string {
+	if e == nil || e.affectedMsg == nil {
+		return ""
+	}
+	return e.affectedMsg.GetMessageID()
+}
+
+// Msg returns the pointer to the affected message that caused the error
+func (e *SendError) Msg() *Msg {
+	if e == nil || e.affectedMsg == nil {
+		return nil
+	}
+	return e.affectedMsg
+}
+
 // String implements the Stringer interface for the SendErrReason
 func (r SendErrReason) String() string {
 	switch r {

senderror_test.go

@@ -83,7 +83,96 @@ func TestSendError_IsTemp(t *testing.T) {
 	}
 }
 
+func TestSendError_IsTempNil(t *testing.T) {
+	var se *SendError
+	if se.IsTemp() {
+		t.Error("expected false on nil-senderror")
+	}
+}
+
+func TestSendError_MessageID(t *testing.T) {
+	var se *SendError
+	err := returnSendError(ErrAmbiguous, false)
+	if !errors.As(err, &se) {
+		t.Errorf("error mismatch, expected error to be of type *SendError")
+		return
+	}
+	if errors.As(err, &se) {
+		if se.MessageID() == "" {
+			t.Errorf("sendError expected message-id, but got empty string")
+		}
+		if !strings.EqualFold(se.MessageID(), "<this.is.a.message.id>") {
+			t.Errorf("sendError message-id expected: %s, but got: %s", "<this.is.a.message.id>",
+				se.MessageID())
+		}
+	}
+}
+
+func TestSendError_MessageIDNil(t *testing.T) {
+	var se *SendError
+	if se.MessageID() != "" {
+		t.Error("expected empty string on nil-senderror")
+	}
+}
+
+func TestSendError_Msg(t *testing.T) {
+	var se *SendError
+	err := returnSendError(ErrAmbiguous, false)
+	if !errors.As(err, &se) {
+		t.Errorf("error mismatch, expected error to be of type *SendError")
+		return
+	}
+	if errors.As(err, &se) {
+		if se.Msg() == nil {
+			t.Errorf("sendError expected msg pointer, but got nil")
+		}
+		from := se.Msg().GetFromString()
+		if len(from) == 0 {
+			t.Errorf("sendError expected msg from, but got empty string")
+			return
+		}
+		if !strings.EqualFold(from[0], "<toni.tester@domain.tld>") {
+			t.Errorf("sendError message from expected: %s, but got: %s", "<toni.tester@domain.tld>",
+				from[0])
+		}
+	}
+}
+
+func TestSendError_MsgNil(t *testing.T) {
+	var se *SendError
+	if se.Msg() != nil {
+		t.Error("expected nil on nil-senderror")
+	}
+}
+
+func TestSendError_IsFail(t *testing.T) {
+	err1 := returnSendError(ErrAmbiguous, false)
+	err2 := returnSendError(ErrSMTPMailFrom, false)
+	if errors.Is(err1, err2) {
+		t.Errorf("error mismatch, ErrAmbiguous should not be equal to ErrSMTPMailFrom")
+	}
+}
+
+func TestSendError_ErrorMulti(t *testing.T) {
+	expected := `ambiguous reason, check Msg.SendError for message specific reasons, ` +
+		`affected recipient(s): <email1@domain.tld>, <email2@domain.tld>`
+	err := &SendError{
+		Reason: ErrAmbiguous, isTemp: false, affectedMsg: nil,
+		rcpt: []string{"<email1@domain.tld>", "<email2@domain.tld>"},
+	}
+	if err.Error() != expected {
+		t.Errorf("error mismatch, expected: %s, got: %s", expected, err.Error())
+	}
+}
+
 // returnSendError is a helper method to retunr a SendError with a specific reason
 func returnSendError(r SendErrReason, t bool) error {
-	return &SendError{Reason: r, isTemp: t}
+	message := NewMsg()
+	_ = message.From("toni.tester@domain.tld")
+	_ = message.To("tina.tester@domain.tld")
+	message.Subject("This is the subject")
+	message.SetBodyString(TypeTextPlain, "This is the message body")
+	message.SetMessageIDWithValue("this.is.a.message.id")
+
+	return &SendError{Reason: r, isTemp: t, affectedMsg: message}
 }