self._log(line)
                 if self._caps.sasl_wait:
                     if self.conn_setup.password:
+                        self._log('trying to authenticate via SASL/plain')
                         self.send(IrcMessage('AUTHENTICATE', ('PLAIN',)))
                     else:
                         self._caps.challenge('END')
                                       ).encode('utf-8')).decode('utf-8')
                     self.send(IrcMessage('AUTHENTICATE', (auth,)))
             case '903' | '904':
-                if msg.verb == '904':
-                    self._log('SASL authentication failed', alert=True)
+                alert = msg.verb == '904'
+                self._log(f'SASL auth {"failed" if alert else "succeeded"}',
+                          alert=alert)
                 self._caps.challenge('END')