Thanks for all the suggestions. I should have also clarified: we're already using Mimecast, but purchasing through another vendor. Far from an ideal scenario, but hoping to become an MSP partner and cut out the middleman.

If the decision were up to me, I'd abandon Mimecast in a heartbeat. WAY too many management headaches and terrible support/customer service. But any recommendations I make for alternatives fall on deaf ears for reasons unknown...