Thought I’d share this for anyone else who runs into the same problem:
I had an After New Mail has arrived agent which wasn’t running as expected. I’d send mail to the database, I could see the agent getting ready to run in the Agent Manager schedule, but then nothing seemed to happen. The Agent Log seemed to suggest that the agent had been triggered, but none of the code was executed and there were no errors anywhere.
It turns out that a Default Form was all that was required (I already had a Default View so I don’t know whether not having one of these would have stopped it from running as well).
So, if you’re having problem with an agent triggered by a Mail event, make sure you’ve got a default form (and probably a default view as well) in your database.
Hope this helps someone!
Emily.