cPanel: 553 Incorrect authentication data

A customer had recently restored their own backup through cPanel, and since doing so they were receiving the following error when trying to send e-mail from cPanel’s webmail (Horde):

553 Incorrect authentication data

They could log in to webmail and receive e-mails fine.

The problem was caused by incorrect mail box permissions. To fix this, simply log in to WHM and go to Email > Repair Mailbox Permissions. If you are an end user and don’t know what WHM is, simply contact your host about the problem and include a link to this post.