Symptoms

Port 25 is closed, so submission port is used. Autoreply does not work. In event viewer:

Log Name:      Plesk
Source:        plesksrv.exe
Date:          8/4/2016 8:55:38 AM
Event ID:      1
Task Category: None
Level:         Warning
Description:
The event class for this subscription is in an invalid partition (COM Error 8004020F, Description: The server rejected one or more recipient addresses. The server response was: 550 This mail server requires authentication (1) when attempting to send via this SMTP (submission) port.)
    at sendMail(plesktest@domain.tld, plesktest1@domain.tld)
    at (MessageProcessor::sendMail line 504)
Execute file name: C:\\Parallels\\Plesk\\admin\\bin\\plesksrv.exe

Cause

Invalid External SMTP Server Settings - username/password is required in case of 587 (submission) port is used.

Resolution

1. Go to Plesk-> Tools & Settings > External SMTP Server Settings
2. Set username and password
3. Click to "OK" button