Fix Quit handling when initial HELO fails

Ensure QUIT command can be sent even if initial HELO fails. Added a check to skip retrying HELO if it already failed, allowing for proper closing of the connection. This prevents potential hangs or errors during connection termination.

smtp/smtp.go

@@ -554,9 +554,20 @@ func (c *Client) Noop() error {
 
 // Quit sends the QUIT command and closes the connection to the server.
 func (c *Client) Quit() error {
+	// If we did already tried to send a EHLO/HELO but it failed, we still need to be able to send
+	// a QUIT to close the connection.
+	// c.hello() will return the global helloErr of the Client, which will always be set if the HELO
+	// failed before. Therefore if we already sent a HELO and the error is not nil, we skip another
+	// EHLO/HELO try
+	c.mutex.RLock()
+	didHello := c.didHello
+	helloErr := c.helloError
+	c.mutex.RUnlock()
+	if !didHello || helloErr == nil {
 		if err := c.hello(); err != nil {
 			return err
 		}
+	}
 	_, _, err := c.cmd(221, "QUIT")
 	if err != nil {
 		return err