msg INBOX/4624 {75129} couldn't append (Subject:[Email message]) to folder INBOX: Error sending '55 APPEND INBOX (\Seen) "25-Aug-2017 09:12:05 -0600" {75129}': 55 NO [OVERQUOTA] Not enough disk quota (0.001 + 0.000 secs).
To resolve the above problem, check the following
- Email mailbox allocated size
- cPanel account user Quota
The above problem was due to the fact that the cPanel User Quota was maxed out. Increasing the space allocated to the account resolved the problem.