Add mutex locks to DSN option setters

Mutex locks are added to ensure thread safety when setting DSN mail return and recipient notify options. This prevents data races in concurrent environments, improving the client's robustness.

smtp/smtp.go

@@ -601,12 +601,16 @@ func (c *Client) SetLogAuthData() {
 
 // SetDSNMailReturnOption sets the DSN mail return option for the Mail method
 func (c *Client) SetDSNMailReturnOption(d string) {
+	c.mutex.Lock()
 	c.dsnmrtype = d
+	c.mutex.Unlock()
 }
 
 // SetDSNRcptNotifyOption sets the DSN recipient notify option for the Mail method
 func (c *Client) SetDSNRcptNotifyOption(d string) {
+	c.mutex.Lock()
 	c.dsnrntype = d
+	c.mutex.Unlock()
 }
 
 // HasConnection checks if the client has an active connection.