user_external: disabling logins with '@' does not work any more

[crysman]

Affected apps

user_external

Expected behaviour

login form should prevent user from logging-in when input contains '@'
(it used to work correctly before)

Actual behaviour

user is logged-in and actually a new user is created (different from the user without @Domain) - see this bug thread: #2219

Steps to reproduce

I've been having this modification of the imap.php file since 2016:

	public function checkPassword($uid, $password) {
		if (!function_exists('imap_open')) {
			OCP\Util::writeLog('user_external', 'ERROR: PHP imap extension is not installed', OCP\Util::ERROR);
			return false;
		}
		$mbox = @imap_open($this->mailbox, $uid, $password, OP_HALFOPEN, 1);
		imap_errors();
		imap_alerts();
		if($mbox !== FALSE) {
			imap_close($mbox);
			$uid = mb_strtolower($uid);
			$this->storeUser($uid);
                /* START added/edited by crysman 2016-05-09 ...*/
                        //protection against whole e-mail input:
                        if( stripos($uid, "@") == false ) {
                                $this->storeUser($uid);
                        } else {
                                return false;
                        }
                        $config = \OC::$server->getConfig();

                        //Set default quota:
                        if($config->getUserValue($uid, 'files', 'quota') === null) {
                          $config->setUserValue($uid, 'files', 'quota', '333 MB');
                        }

                        //Set the IMAP user's email to their login:
                        $config->setUserValue( $uid, 'settings', 'email', $uid.'@mydomain.cz');

                        //Add a new user automatically to "SomeFantasticGroup" group:
                        $user = \OC::$server->getUserSession()->getUser();
                        if($user !== null) {
                          $group = \OC::$server->getGroupManager()->get('SomeFantasticGroup');
                          if(!$group->inGroup($user)) {
                            $group->addUser($user);
                          }
                        }
                /* END crysman*/
			return $uid;
		}else{
			return false;
		}
	}

recently it has stopped working (I do not know since when exactly) - why? As you can see, the piece of code after //protection against whole e-mail input: should prevent the user from logging.
How to modify it to make it work again? Thank you.

Server configuration

Operating system:
Debian 8
Web server:
apache2
Database:
mysql
PHP version:
5.6.30
ownCloud version:
9.1.6