Symptoms
When sending email to an account on your server, or when performing a callback verification the following error message is provided:
550 5.1.1 User unknown: user@domain.tld
Description
The ASSP software can create a problem where users are not detected. The ASSP software is not created by, provided by, or supported by cPanel.
In order to properly diagnose and resolve the issue, you must reach out to ASSP support:
http://www.thockar.com/assp-home/
Workaround
You may disable the ASSP plugin to workaround this issue.