Additionally the following errors may be shown:
Failed to connect to mailserver
SMTP server response: 550
Empty error message from utility.
CauseThis error means that Plesk failed to send notification for some reasons.
First of all make sure that Plesk SMTP server functioning properly as described in 1387.
Also, check Plesk Notifications configuration on the Server->Notification page in Plesk CP. Make sure that all mail addresses notification are configured to sent to are valid and there are no empty Email fields with enabled notifications checkbox.