I just got off the phone with Cartama. BTW, they are charging us about $25 a month to host this site - and we are actually a fairly "large" site - not just a simple company or club website.

He said the server we are on is overloaded at certain times of day, so he's offloading an email vm tonight, to another server. This should happen tonight. That should cure the problem. He'll monitor it for the next few weeks, and see if the problem goes away.