Merge pull request #302 from wneessen/feature/improve-client-tests

Improved client error testing

.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

client_119.go

@@ -7,16 +7,22 @@
 
 package mail
 
+import "errors"
+
 // Send sends out the mail message
 func (c *Client) Send(messages ...*Msg) error {
 	if err := c.checkConn(); err != nil {
 		return &SendError{Reason: ErrConnCheck, errlist: []error{err}, isTemp: isTempError(err)}
 	}
 	var errs []*SendError
-	for _, message := range messages {
+	for id, message := range messages {
 		if sendErr := c.sendSingleMsg(message); sendErr != nil {
 			messages[id].sendError = sendErr
-			errs = append(errs, sendErr)
+
+			var msgSendErr *SendError
+			if errors.As(sendErr, &msgSendErr) {
+				errs = append(errs, msgSendErr)
+			}
 		}
 	}
 

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"
 )
 
-// DefaultHost is used as default hostname for the Client
-const DefaultHost = "localhost"
-
-// TestRcpt
-const TestRcpt = "go-mail@mytrashmailer.com"
+const (
+	// DefaultHost is used as default hostname for the Client
+	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) {
@@ -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) {
@@ -1545,3 +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")
+		}
+	}
+}

senderror.go

@@ -118,6 +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

@@ -90,7 +90,55 @@ func TestSendError_IsTempNil(t *testing.T) {
 	}
 }
 
+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_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])
+		}
+	}
+}
+
 // 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}
 }