Given the ClamAV issue, it seemed I had no choice but to upgrade, so, I upgraded from 5.0.16 to 5.0.23.
This appeared to go smoothly, however, once the update was complete, I was unable to start mailbox. Specifically, zmcontrol start completes without any error messages, but zmcontrol status shows nothing running. I've got no extra server running, and nothing binding to any of the common ports.
Any help would be much appreciated.