I think I found a bug, since I just did a clean install and migrate my old customers from 12.0 to Onyx. So I will follow the bug format and explain what I did to fix it.
Plesk Onyx, Version 17.0.17 Update #7, CentOS Linux 7.2.1511 (Core), x86_64
PROBLEM DESCRIPTION: I had a few mailbox with...