Lock sendMutex to ensure thread safety in Send

Added a mutex lock and unlock around the Send function to prevent concurrent access issues and ensure thread safety. This change helps avoid race conditions when multiple goroutines attempt to send messages simultaneously.

client_120.go

@@ -29,6 +29,8 @@ import (
 // Returns:
 //   - An error that aggregates any SendErrors encountered during the sending process; otherwise, returns nil.
 func (c *Client) Send(messages ...*Msg) (returnErr error) {
+	c.sendMutex.Lock()
+	defer c.sendMutex.Unlock()
 	return c.SendWithSMTPClient(c.smtpClient, messages...)
 }