Every couple of weeks our antispam/virus scanning server shuts down for now reason. At about 8:00 AM it went offline after being up 9 days. I had to rebuild some databases and brought it back online around 8:35 AM. At present all mail has been delivered. During this outage outbound mail was not possible unless using the webmail.
Mail volume has nearly doubled in the past month from 65,000 messages per day to 122,000 messages per day. Maybe it's a volume issue.