Add concurrent send tests for Client

Introduced TestClient_DialSendConcurrent_online and TestClient_DialSendConcurrent_local to validate concurrent sending of messages. These tests ensure that the Client's send functionality works correctly under concurrent conditions, both in an online environment and using a local test server.

.github/workflows/codeql-analysis.yml

@@ -54,7 +54,7 @@ jobs:
 
     # Initializes the CodeQL tools for scanning.
     - name: Initialize CodeQL
-      uses: github/codeql-action/init@294a9d92911152fe08befb9ec03e240add280cb3 # v3.26.8
+      uses: github/codeql-action/init@461ef6c76dfe95d5c364de2f431ddbd31a417628 # v3.26.9
       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@294a9d92911152fe08befb9ec03e240add280cb3 # v3.26.8
+      uses: github/codeql-action/autobuild@461ef6c76dfe95d5c364de2f431ddbd31a417628 # v3.26.9
 
     # 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@294a9d92911152fe08befb9ec03e240add280cb3 # v3.26.8
+      uses: github/codeql-action/analyze@461ef6c76dfe95d5c364de2f431ddbd31a417628 # v3.26.9

.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@294a9d92911152fe08befb9ec03e240add280cb3 # v3.26.8
+        uses: github/codeql-action/upload-sarif@461ef6c76dfe95d5c364de2f431ddbd31a417628 # v3.26.9
         with:
           sarif_file: results.sarif

.github/workflows/sonarqube.yml

@@ -44,7 +44,7 @@ jobs:
         run: |
           go test -v -race --coverprofile=./cov.out ./...
 
-      - uses: sonarsource/sonarqube-scan-action@0c0f3958d90fc466625f1d1af1f47bddd4cc6bd1 # master
+      - uses: sonarsource/sonarqube-scan-action@884b79409bbd464b2a59edc326a4b77dc56b2195 # master
         env:
           SONAR_TOKEN: ${{ secrets.SONAR_TOKEN }}
           SONAR_HOST_URL: ${{ secrets.SONAR_HOST_URL }}

client.go

@@ -88,13 +88,12 @@ type DialContextFunc func(ctx context.Context, network, address string) (net.Con
 
 // Client is the SMTP client struct
 type Client struct {
-	mutex sync.RWMutex
-	// connection is the net.Conn that the smtp.Client is based on
-	connection net.Conn
-
 	// Timeout for the SMTP server connection
 	connTimeout time.Duration
 
+	// dialContextFunc is a custom DialContext function to dial target SMTP server
+	dialContextFunc DialContextFunc
+
 	// dsn indicates that we want to use DSN for the Client
 	dsn bool
 
@@ -104,11 +103,9 @@ type Client struct {
 	// dsnrntype defines the DSNRcptNotifyOption in case DSN is enabled
 	dsnrntype []string
 
-	// isEncrypted indicates if a Client connection is encrypted or not
+	// fallbackPort is used as an alternative port number in case the primary port is unavailable or
-	isEncrypted bool
+	// fails to bind.
-
+	fallbackPort int
-	// noNoop indicates the Noop is to be skipped
-	noNoop bool
 
 	// HELO/EHLO string for the greeting the target SMTP server
 	helo string
@@ -116,12 +113,24 @@ type Client struct {
 	// Hostname of the target SMTP server to connect to
 	host string
 
+	// isEncrypted indicates if a Client connection is encrypted or not
+	isEncrypted bool
+
+	// logger is a logger that implements the log.Logger interface
+	logger log.Logger
+
+	// mutex is used to synchronize access to shared resources, ensuring that only one goroutine can
+	// modify them at a time.
+	mutex sync.RWMutex
+
+	// noNoop indicates the Noop is to be skipped
+	noNoop bool
+
 	// pass is the corresponding SMTP AUTH password
 	pass string
 
-	// Port of the SMTP server to connect to
+	// port specifies the network port number on which the server listens for incoming connections.
 	port int
-	fallbackPort int
 
 	// smtpAuth is a pointer to smtp.Auth
 	smtpAuth smtp.Auth
@@ -132,26 +141,20 @@ type Client struct {
 	// smtpClient is the smtp.Client that is set up when using the Dial*() methods
 	smtpClient *smtp.Client
 
-	// Use SSL for the connection
-	useSSL bool
-
 	// tlspolicy sets the client to use the provided TLSPolicy for the STARTTLS protocol
 	tlspolicy TLSPolicy
 
 	// tlsconfig represents the tls.Config setting for the STARTTLS connection
 	tlsconfig *tls.Config
 
-	// user is the SMTP AUTH username
-	user string
-
 	// useDebugLog enables the debug logging on the SMTP client
 	useDebugLog bool
 
-	// logger is a logger that implements the log.Logger interface
+	// user is the SMTP AUTH username
-	logger log.Logger
+	user string
 
-	// dialContextFunc is a custom DialContext function to dial target SMTP server
+	// Use SSL for the connection
-	dialContextFunc DialContextFunc
+	useSSL bool
 }
 
 // Option returns a function that can be used for grouping Client options
@@ -552,6 +555,9 @@ func (c *Client) SetLogger(logger log.Logger) {
 
 // SetTLSConfig overrides the current *tls.Config with the given *tls.Config value
 func (c *Client) SetTLSConfig(tlsconfig *tls.Config) error {
+	c.mutex.Lock()
+	defer c.mutex.Unlock()
+
 	if tlsconfig == nil {
 		return ErrInvalidTLSConfig
 	}
@@ -795,6 +801,9 @@ func (c *Client) auth() error {
 // 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 {
+	c.mutex.Lock()
+	defer c.mutex.Unlock()
+
 	if message.encoding == NoEncoding {
 		if ok, _ := c.smtpClient.Extension("8BITMIME"); !ok {
 			return &SendError{Reason: ErrNoUnencoded, isTemp: false, affectedMsg: message}

client_120.go

@@ -13,8 +13,6 @@ import (
 
 // Send sends out the mail message
 func (c *Client) Send(messages ...*Msg) (returnErr error) {
-	c.mutex.Lock()
-	defer c.mutex.Unlock()
 	if err := c.checkConn(); err != nil {
 		returnErr = &SendError{Reason: ErrConnCheck, errlist: []error{err}, isTemp: isTempError(err)}
 		return

client_test.go

@@ -15,6 +15,7 @@ import (
 	"os"
 	"strconv"
 	"strings"
+	"sync"
 	"testing"
 	"time"
 
@@ -623,11 +624,12 @@ func TestClient_DialWithContext(t *testing.T) {
 		t.Errorf("failed to dial with context: %s", err)
 		return
 	}
-	if c.connection == nil {
-		t.Errorf("DialWithContext didn't fail but no connection found.")
-	}
 	if c.smtpClient == nil {
 		t.Errorf("DialWithContext didn't fail but no SMTP client found.")
+		return
+	}
+	if !c.smtpClient.HasConnection() {
+		t.Errorf("DialWithContext didn't fail but no connection found.")
 	}
 	if err := c.Close(); err != nil {
 		t.Errorf("failed to close connection: %s", err)
@@ -644,17 +646,18 @@ func TestClient_DialWithContext_Fallback(t *testing.T) {
 	c.SetTLSPortPolicy(TLSOpportunistic)
 	c.port = 999
 	ctx := context.Background()
-	if err := c.DialWithContext(ctx); err != nil {
+	if err = c.DialWithContext(ctx); err != nil {
 		t.Errorf("failed to dial with context: %s", err)
 		return
 	}
-	if c.connection == nil {
-		t.Errorf("DialWithContext didn't fail but no connection found.")
-	}
 	if c.smtpClient == nil {
 		t.Errorf("DialWithContext didn't fail but no SMTP client found.")
+		return
 	}
-	if err := c.Close(); err != nil {
+	if !c.smtpClient.HasConnection() {
+		t.Errorf("DialWithContext didn't fail but no connection found.")
+	}
+	if err = c.Close(); err != nil {
 		t.Errorf("failed to close connection: %s", err)
 	}
 
@@ -674,18 +677,19 @@ func TestClient_DialWithContext_Debug(t *testing.T) {
 		t.Skipf("failed to create test client: %s. Skipping tests", err)
 	}
 	ctx := context.Background()
-	if err := c.DialWithContext(ctx); err != nil {
+	if err = c.DialWithContext(ctx); err != nil {
 		t.Errorf("failed to dial with context: %s", err)
 		return
 	}
-	if c.connection == nil {
-		t.Errorf("DialWithContext didn't fail but no connection found.")
-	}
 	if c.smtpClient == nil {
 		t.Errorf("DialWithContext didn't fail but no SMTP client found.")
+		return
+	}
+	if !c.smtpClient.HasConnection() {
+		t.Errorf("DialWithContext didn't fail but no connection found.")
 	}
 	c.SetDebugLog(true)
-	if err := c.Close(); err != nil {
+	if err = c.Close(); err != nil {
 		t.Errorf("failed to close connection: %s", err)
 	}
 }
@@ -698,19 +702,20 @@ func TestClient_DialWithContext_Debug_custom(t *testing.T) {
 		t.Skipf("failed to create test client: %s. Skipping tests", err)
 	}
 	ctx := context.Background()
-	if err := c.DialWithContext(ctx); err != nil {
+	if err = c.DialWithContext(ctx); err != nil {
 		t.Errorf("failed to dial with context: %s", err)
 		return
 	}
-	if c.connection == nil {
-		t.Errorf("DialWithContext didn't fail but no connection found.")
-	}
 	if c.smtpClient == nil {
 		t.Errorf("DialWithContext didn't fail but no SMTP client found.")
+		return
+	}
+	if !c.smtpClient.HasConnection() {
+		t.Errorf("DialWithContext didn't fail but no connection found.")
 	}
 	c.SetDebugLog(true)
 	c.SetLogger(log.New(os.Stderr, log.LevelDebug))
-	if err := c.Close(); err != nil {
+	if err = c.Close(); err != nil {
 		t.Errorf("failed to close connection: %s", err)
 	}
 }
@@ -722,10 +727,9 @@ func TestClient_DialWithContextInvalidHost(t *testing.T) {
 	if err != nil {
 		t.Skipf("failed to create test client: %s. Skipping tests", err)
 	}
-	c.connection = nil
 	c.host = "invalid.addr"
 	ctx := context.Background()
-	if err := c.DialWithContext(ctx); err == nil {
+	if err = c.DialWithContext(ctx); err == nil {
 		t.Errorf("dial succeeded but was supposed to fail")
 		return
 	}
@@ -738,10 +742,9 @@ func TestClient_DialWithContextInvalidHELO(t *testing.T) {
 	if err != nil {
 		t.Skipf("failed to create test client: %s. Skipping tests", err)
 	}
-	c.connection = nil
 	c.helo = ""
 	ctx := context.Background()
-	if err := c.DialWithContext(ctx); err == nil {
+	if err = c.DialWithContext(ctx); err == nil {
 		t.Errorf("dial succeeded but was supposed to fail")
 		return
 	}
@@ -758,7 +761,7 @@ func TestClient_DialWithContextInvalidAuth(t *testing.T) {
 	c.pass = "invalid"
 	c.SetSMTPAuthCustom(smtp.LoginAuth("invalid", "invalid", "invalid"))
 	ctx := context.Background()
-	if err := c.DialWithContext(ctx); err == nil {
+	if err = c.DialWithContext(ctx); err == nil {
 		t.Errorf("dial succeeded but was supposed to fail")
 		return
 	}
@@ -770,8 +773,7 @@ func TestClient_checkConn(t *testing.T) {
 	if err != nil {
 		t.Skipf("failed to create test client: %s. Skipping tests", err)
 	}
-	c.connection = nil
+	if err = c.checkConn(); err == nil {
-	if err := c.checkConn(); err == nil {
 		t.Errorf("connCheck() should fail but succeeded")
 	}
 }
@@ -802,21 +804,23 @@ func TestClient_DialWithContextOptions(t *testing.T) {
 			}
 
 			ctx := context.Background()
-			if err := c.DialWithContext(ctx); err != nil && !tt.sf {
+			if err = c.DialWithContext(ctx); err != nil && !tt.sf {
 				t.Errorf("failed to dial with context: %s", err)
 				return
 			}
 			if !tt.sf {
-				if c.connection == nil && !tt.sf {
-					t.Errorf("DialWithContext didn't fail but no connection found.")
-				}
 				if c.smtpClient == nil && !tt.sf {
 					t.Errorf("DialWithContext didn't fail but no SMTP client found.")
+					return
 				}
-				if err := c.Reset(); err != nil {
+				if !c.smtpClient.HasConnection() && !tt.sf {
+					t.Errorf("DialWithContext didn't fail but no connection found.")
+					return
+				}
+				if err = c.Reset(); err != nil {
 					t.Errorf("failed to reset connection: %s", err)
 				}
-				if err := c.Close(); err != nil {
+				if err = c.Close(); err != nil {
 					t.Errorf("failed to close connection: %s", err)
 				}
 			}
@@ -1011,17 +1015,15 @@ func TestClient_DialSendCloseBroken(t *testing.T) {
 			}
 			if tt.closestart {
 				_ = c.smtpClient.Close()
-				_ = c.connection.Close()
 			}
-			if err := c.Send(m); err != nil && !tt.sf {
+			if err = c.Send(m); err != nil && !tt.sf {
 				t.Errorf("Send() failed: %s", err)
 				return
 			}
 			if tt.closeearly {
 				_ = c.smtpClient.Close()
-				_ = c.connection.Close()
 			}
-			if err := c.Close(); err != nil && !tt.sf {
+			if err = c.Close(); err != nil && !tt.sf {
 				t.Errorf("Close() failed: %s", err)
 				return
 			}
@@ -1071,17 +1073,15 @@ func TestClient_DialSendCloseBrokenWithDSN(t *testing.T) {
 			}
 			if tt.closestart {
 				_ = c.smtpClient.Close()
-				_ = c.connection.Close()
 			}
-			if err := c.Send(m); err != nil && !tt.sf {
+			if err = c.Send(m); err != nil && !tt.sf {
 				t.Errorf("Send() failed: %s", err)
 				return
 			}
 			if tt.closeearly {
 				_ = c.smtpClient.Close()
-				_ = c.connection.Close()
 			}
-			if err := c.Close(); err != nil && !tt.sf {
+			if err = c.Close(); err != nil && !tt.sf {
 				t.Errorf("Close() failed: %s", err)
 				return
 			}
@@ -1728,6 +1728,114 @@ func TestClient_SendErrorReset(t *testing.T) {
 	}
 }
 
+func TestClient_DialSendConcurrent_online(t *testing.T) {
+	if os.Getenv("TEST_ALLOW_SEND") == "" {
+		t.Skipf("TEST_ALLOW_SEND is not set. Skipping mail sending test")
+	}
+
+	client, err := getTestConnection(true)
+	if err != nil {
+		t.Errorf("unable to create new client: %s", err)
+	}
+
+	var messages []*Msg
+	for i := 0; i < 10; i++ {
+		message := NewMsg()
+		if err := message.FromFormat("go-mail Test Mailer", os.Getenv("TEST_FROM")); err != nil {
+			t.Errorf("failed to set FROM address: %s", err)
+			return
+		}
+		if err := message.To(TestRcpt); err != nil {
+			t.Errorf("failed to set TO address: %s", err)
+			return
+		}
+		message.Subject(fmt.Sprintf("Test subject for mail %d", i))
+		message.SetBodyString(TypeTextPlain, fmt.Sprintf("This is the test body of the mail no. %d", i))
+		message.SetMessageID()
+		messages = append(messages, message)
+	}
+
+	if err = client.DialWithContext(context.Background()); err != nil {
+		t.Errorf("failed to dial to test server: %s", err)
+	}
+
+	wg := sync.WaitGroup{}
+	for id, message := range messages {
+		wg.Add(1)
+		go func(curMsg *Msg, curID int) {
+			defer wg.Done()
+			if goroutineErr := client.Send(curMsg); err != nil {
+				t.Errorf("failed to send message with ID %d: %s", curID, goroutineErr)
+			}
+		}(message, id)
+	}
+	wg.Wait()
+
+	if err = client.Close(); err != nil {
+		t.Errorf("failed to close server connection: %s", err)
+	}
+}
+
+func TestClient_DialSendConcurrent_local(t *testing.T) {
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	serverPort := TestServerPortBase + 20
+	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 * 500)
+
+	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)
+	}
+
+	var messages []*Msg
+	for i := 0; i < 50; i++ {
+		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")
+		messages = append(messages, message)
+	}
+
+	if err = client.DialWithContext(context.Background()); err != nil {
+		t.Errorf("failed to dial to test server: %s", err)
+	}
+
+	wg := sync.WaitGroup{}
+	for id, message := range messages {
+		wg.Add(1)
+		go func(curMsg *Msg, curID int) {
+			defer wg.Done()
+			if goroutineErr := client.Send(curMsg); err != nil {
+				t.Errorf("failed to send message with ID %d: %s", curID, goroutineErr)
+			}
+		}(message, id)
+	}
+	wg.Wait()
+
+	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) {
@@ -2100,6 +2208,7 @@ func handleTestServerConnection(connection net.Conn, featureSet string, failRese
 		if err != nil {
 			break
 		}
+		time.Sleep(time.Millisecond)
 
 		var datastring string
 		data = strings.TrimSpace(data)

random_119.go

@@ -2,8 +2,8 @@
 //
 // SPDX-License-Identifier: MIT
 
-//go:build go1.19 && !go1.20
+//go:build !go1.20
-// +build go1.19,!go1.20
+// +build !go1.20
 
 package mail
 

smtp/smtp.go

@@ -30,6 +30,7 @@ import (
 	"net/textproto"
 	"os"
 	"strings"
+	"sync"
 	"time"
 
 	"github.com/wneessen/go-mail/log"
@@ -37,28 +38,49 @@ import (
 
 // A Client represents a client connection to an SMTP server.
 type Client struct {
-	// Text is the textproto.Conn used by the Client. It is exported to allow for
+	// Text is the textproto.Conn used by the Client. It is exported to allow for clients to add extensions.
-	// clients to add extensions.
 	Text *textproto.Conn
-	// keep a reference to the connection so it can be used to create a TLS
+
-	// connection later
+	// auth supported auth mechanisms
-	conn net.Conn
-	// whether the Client is using TLS
-	tls        bool
-	serverName string
-	// map of supported extensions
-	ext map[string]string
-	// supported auth mechanisms
 	auth []string
+
+	// keep a reference to the connection so it can be used to create a TLS connection later
+	conn net.Conn
+
+	// debug logging is enabled
+	debug bool
+
+	// didHello indicates whether we've said HELO/EHLO
+	didHello bool
+
+	// dsnmrtype defines the mail return option in case DSN is enabled
+	dsnmrtype string
+
+	// dsnrntype defines the recipient notify option in case DSN is enabled
+	dsnrntype string
+
+	// ext is a map of supported extensions
+	ext map[string]string
+
+	// helloError is the error from the hello
+	helloError error
+
+	// localName is the name to use in HELO/EHLO
 	localName string // the name to use in HELO/EHLO
-	didHello   bool   // whether we've said HELO/EHLO
+
-	helloError error  // the error from the hello
+	// logger will be used for debug logging
-	// debug logging
+	logger log.Logger
-	debug  bool       // debug logging is enabled
+
-	logger log.Logger // logger will be used for debug logging
+	// mutex is used to synchronize access to shared resources, ensuring that only one goroutine can access
-	// DSN support
+	// the resource at a time.
-	dsnmrtype string // dsnmrtype defines the mail return option in case DSN is enabled
+	mutex sync.RWMutex
-	dsnrntype string // dsnrntype defines the recipient notify option in case DSN is enabled
+
+	// tls indicates whether the Client is using TLS
+	tls bool
+
+	// serverName denotes the name of the server to which the application will connect. Used for
+	// identification and routing.
+	serverName string
 }
 
 // Dial returns a new [Client] connected to an SMTP server at addr.
@@ -95,7 +117,10 @@ func NewClient(conn net.Conn, host string) (*Client, error) {
 
 // Close closes the connection.
 func (c *Client) Close() error {
-	return c.Text.Close()
+	c.mutex.Lock()
+	err := c.Text.Close()
+	c.mutex.Unlock()
+	return err
 }
 
 // hello runs a hello exchange if needed.
@@ -122,28 +147,39 @@ func (c *Client) Hello(localName string) error {
 	if c.didHello {
 		return errors.New("smtp: Hello called after other methods")
 	}
+
+	c.mutex.Lock()
 	c.localName = localName
+	c.mutex.Unlock()
+
 	return c.hello()
 }
 
 // cmd is a convenience function that sends a command and returns the response
 func (c *Client) cmd(expectCode int, format string, args ...interface{}) (int, string, error) {
+	c.mutex.Lock()
+
 	c.debugLog(log.DirClientToServer, format, args...)
 	id, err := c.Text.Cmd(format, args...)
 	if err != nil {
+		c.mutex.Unlock()
 		return 0, "", err
 	}
 	c.Text.StartResponse(id)
-	defer c.Text.EndResponse(id)
 	code, msg, err := c.Text.ReadResponse(expectCode)
 	c.debugLog(log.DirServerToClient, "%d %s", code, msg)
+	c.Text.EndResponse(id)
+	c.mutex.Unlock()
 	return code, msg, err
 }
 
 // helo sends the HELO greeting to the server. It should be used only when the
 // server does not support ehlo.
 func (c *Client) helo() error {
+	c.mutex.Lock()
 	c.ext = nil
+	c.mutex.Unlock()
+
 	_, _, err := c.cmd(250, "HELO %s", c.localName)
 	return err
 }
@@ -158,9 +194,13 @@ func (c *Client) StartTLS(config *tls.Config) error {
 	if err != nil {
 		return err
 	}
+
+	c.mutex.Lock()
 	c.conn = tls.Client(c.conn, config)
 	c.Text = textproto.NewConn(c.conn)
 	c.tls = true
+	c.mutex.Unlock()
+
 	return c.ehlo()
 }
 
@@ -168,11 +208,15 @@ func (c *Client) StartTLS(config *tls.Config) error {
 // The return values are their zero values if [Client.StartTLS] did
 // not succeed.
 func (c *Client) TLSConnectionState() (state tls.ConnectionState, ok bool) {
+	c.mutex.RLock()
+
 	tc, ok := c.conn.(*tls.Conn)
 	if !ok {
 		return
 	}
-	return tc.ConnectionState(), true
+	state, ok = tc.ConnectionState(), true
+	c.mutex.RUnlock()
+	return
 }
 
 // Verify checks the validity of an email address on the server.
@@ -258,6 +302,8 @@ func (c *Client) Mail(from string) error {
 		return err
 	}
 	cmdStr := "MAIL FROM:<%s>"
+
+	c.mutex.RLock()
 	if c.ext != nil {
 		if _, ok := c.ext["8BITMIME"]; ok {
 			cmdStr += " BODY=8BITMIME"
@@ -270,6 +316,8 @@ func (c *Client) Mail(from string) error {
 			cmdStr += fmt.Sprintf(" RET=%s", c.dsnmrtype)
 		}
 	}
+	c.mutex.RUnlock()
+
 	_, _, err := c.cmd(250, cmdStr, from)
 	return err
 }
@@ -281,7 +329,11 @@ func (c *Client) Rcpt(to string) error {
 	if err := validateLine(to); err != nil {
 		return err
 	}
+
+	c.mutex.RLock()
 	_, ok := c.ext["DSN"]
+	c.mutex.RUnlock()
+
 	if ok && c.dsnrntype != "" {
 		_, _, err := c.cmd(25, "RCPT TO:<%s> NOTIFY=%s", to, c.dsnrntype)
 		return err
@@ -295,12 +347,23 @@ type dataCloser struct {
 	io.WriteCloser
 }
 
+// Close releases the lock, closes the WriteCloser, waits for a response, and then returns any error encountered.
 func (d *dataCloser) Close() error {
+	d.c.mutex.Lock()
 	_ = d.WriteCloser.Close()
 	_, _, err := d.c.Text.ReadResponse(250)
+	d.c.mutex.Unlock()
 	return err
 }
 
+// Write writes data to the underlying WriteCloser while ensuring thread-safety by locking and unlocking a mutex.
+func (d *dataCloser) Write(p []byte) (n int, err error) {
+	d.c.mutex.Lock()
+	n, err = d.WriteCloser.Write(p)
+	d.c.mutex.Unlock()
+	return
+}
+
 // Data issues a DATA command to the server and returns a writer that
 // can be used to write the mail headers and body. The caller should
 // close the writer before calling any more methods on c. A call to
@@ -310,7 +373,14 @@ func (c *Client) Data() (io.WriteCloser, error) {
 	if err != nil {
 		return nil, err
 	}
-	return &dataCloser{c, c.Text.DotWriter()}, nil
+	datacloser := &dataCloser{}
+
+	c.mutex.Lock()
+	datacloser.c = c
+	datacloser.WriteCloser = c.Text.DotWriter()
+	c.mutex.Unlock()
+
+	return datacloser, nil
 }
 
 var testHookStartTLS func(*tls.Config) // nil, except for tests
@@ -406,7 +476,10 @@ func (c *Client) Extension(ext string) (bool, string) {
 		return false, ""
 	}
 	ext = strings.ToUpper(ext)
+
+	c.mutex.RLock()
 	param, ok := c.ext[ext]
+	c.mutex.RUnlock()
 	return ok, param
 }
 
@@ -439,7 +512,11 @@ func (c *Client) Quit() error {
 	if err != nil {
 		return err
 	}
-	return c.Text.Close()
+	c.mutex.Lock()
+	err = c.Text.Close()
+	c.mutex.Unlock()
+
+	return err
 }
 
 // SetDebugLog enables the debug logging for incoming and outgoing SMTP messages
@@ -480,9 +557,11 @@ func (c *Client) HasConnection() bool {
 }
 
 func (c *Client) UpdateDeadline(timeout time.Duration) error {
+	c.mutex.Lock()
 	if err := c.conn.SetDeadline(time.Now().Add(timeout)); err != nil {
 		return fmt.Errorf("smtp: failed to update deadline: %w", err)
 	}
+	c.mutex.Unlock()
 	return nil
 }
 

smtp/smtp_ehlo.go

@@ -25,6 +25,9 @@ func (c *Client) ehlo() error {
 	if err != nil {
 		return err
 	}
+
+	c.mutex.Lock()
+	defer c.mutex.Unlock()
 	ext := make(map[string]string)
 	extList := strings.Split(msg, "\n")
 	if len(extList) > 1 {

smtp/smtp_ehlo_117.go

@@ -28,6 +28,9 @@ func (c *Client) ehlo() error {
 	if err != nil {
 		return err
 	}
+
+	c.mutex.Lock()
+	defer c.mutex.Unlock()
 	ext := make(map[string]string)
 	extList := strings.Split(msg, "\n")
 	if len(extList) > 1 {