Mail stopped going out last night. What happened?
I sent email to support@comodo.com
Lou
Follow up: Both domains are using smarthost again. It seems that neither Comodo or the 2 domains (Exchange servers) made any changes to cause it to stop. And it started working again when I tested it this morning. Support was responsive, I have no clue what caused it. I told support that they could blame me and I would blame them. 
The good take away from this is that I made sure there was a second Exchange send connector, with a higher cost than the smarthost connector. And made sure that it worked.
Lou