Limit the number of maximum connect attempts to 1

http://php.net/manual/en/function.imap-open.php

lib/imap.php

@@ -42,7 +42,7 @@ class OC_User_IMAP extends \OCA\user_external\Base {
 			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);
+		$mbox = @imap_open($this->mailbox, $uid, $password, OP_HALFOPEN, 1);
 		imap_errors();
 		imap_alerts();
 		if($mbox !== FALSE) {