The Zimbra upgrade is currently taking place.  Unfortunately we found an error that caused some emails not to be queued by our MX clusters, instead bouncing incoming emails.  These bounces occurred between 17:00 and 18:30 GMT.  The cause was a broken MTA service on one of our Zimbra MTA servers.  This service has been shutdown and we'll be looking into how it got into this state.

Valid senders will receive bounce emails for any emails they sent during this time.

Gateway scanning services are unaffected.

Apologies for the trouble.