SEM stops sending emails on rule trigger

I've got a run to send an email for EventType = UserEnable. We get around 120 of these a day, so it acts as a decent flag as to whether email alerts are working.

What's happening is around once a week SEM stops sending emails at all until I restart the manager via a PuTTY session. Once I do this all is well again.

We're on the latest edition of SEM and are running the appliance.

How do I begin to trace down what's going on? The rules are triggering and claim they've sent mail, but nothing appears.

The appliance isn't over-utilized at all, taking in only 600-700 EPS.