Fix deferred mutex unlock in TLSConnectionState

Correct the sequence of mutex unlocking in TLSConnectionState to ensure the mutex is always released properly. This prevents potential deadlocks and ensures the function behaves as expected in a concurrent context.

smtp/smtp.go

@@ -209,13 +209,13 @@ func (c *Client) StartTLS(config *tls.Config) error {
 // not succeed.
 func (c *Client) TLSConnectionState() (state tls.ConnectionState, ok bool) {
 	c.mutex.RLock()
+	defer c.mutex.RUnlock()
 
 	tc, ok := c.conn.(*tls.Conn)
 	if !ok {
 		return
 	}
 	state, ok = tc.ConnectionState(), true
-	c.mutex.RUnlock()
 	return
 }