Stuff…

OS: CentOS 5.4
CP: Plesk 9.3
MTA: Postfix

Mail was bouncing when being sent to a mailbox.  The error message was not very helpful at all:

Mar 24 15:28:02 hostname postfix/pipe[14810]: 4E28B167911E: to=<user@domain.org>, relay=plesk_virtual, delay=2.7, delays=2.7/0/0/0.01, dsn=5.7.1, status=bounced (Message can’t be delivered )

Creating a new mailbox on the domain allowed emails to be sent to it fine, so, it was not a problem with mail for the domain on a whole.  It turns out that in this situation it maybe a quota on the mailbox which has been exceeded.

In previous Plesk versions it would mention that the quota was exceeded, but, not in this case.