Move mutex lock to sendSingleMsg method

Mutex locking was relocated from the Send method in client_120.go and client_119.go to sendSingleMsg in client.go. This ensures thread-safety specifically during the message transmission process.

client.go

@@ -801,6 +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_119.go

@@ -11,9 +11,6 @@ import "errors"
 
 // Send sends out the mail message
 func (c *Client) Send(messages ...*Msg) error {
-	c.mutex.Lock()
-	defer c.mutex.Unlock()
-
 	if err := c.checkConn(); err != nil {
 		return &SendError{Reason: ErrConnCheck, errlist: []error{err}, isTemp: isTempError(err)}
 	}

client_120.go

@@ -13,9 +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