Occasionally, after a migration from another server, the mailbox permissions for your users may be incorrect. This can cause issues with sending and/or receiving email.
1. Log in to WHM
2. Click on "Repair Mailbox Permissions"
3. Press "Proceed"
This interface will inspect mailbox ownership and file permissions. You can also attempt to repair any contradictions that may exist in the mail system manually. If this permission fix script fails, the permissions may be unable to be changed in this way. Please see this article for steps on how to proceed further: Mailbox permissions for firstname.lastname@example.org are broken and can't be fixed
This is also available as a script to run via the command line. Please see this article for more information: How to Fix Mailbox Permissions via Command-Line