In rare cases we have seen customers who use Plesk Hosting with 451 Locked Mailbox Email errors. At this point our staff has linked this issue to a message being stuck in the Client Outbox of the email user experiencing the problem.
To resolve this issue the user should remove any email messages from their outbox having trouble sending. This can be caused by corrupt or malformed email messages as well that are not accepted by Qmail within Plesk.
Other symptoms may be an error message upon accessing webmail that indicate there is no connection to the server or that the mailbox may be on lock due to an unidentified issue.
For additional help with this issue please Create a Trouble Ticket using our ticket system.