fix failed login even when it is authenticated in PLAIN authentication

Author: ip-qi

system/Email/Email.php

@@ -2058,10 +2058,13 @@ protected function SMTPAuthenticate()
                 $authString = "\0" . $this->SMTPUser . "\0" . $this->SMTPPass;
 
                 $this->sendData('AUTH PLAIN ' . base64_encode($authString));
-                $reply = $this->getSMTPData();
 
-                if (! str_starts_with($reply, '235') || ! str_starts_with($reply, '503')) {
-                    $this->setErrorMessage(lang('Email.failedSMTPLogin', [$reply]));
+                if (str_starts_with($this->getSMTPData(), '503')) {    // Already authenticated
+                    return true;
+                }
+
+                if (! str_starts_with($this->getSMTPData(), '235')) {
+                    $this->setErrorMessage(lang('Email.failedSMTPLogin', [$this->getSMTPData()]));
 
                     return false;
                 }