We currently limit the maximum number of connections to 100. However, we still have occasional excessive sender get rejected when they exceed it. My question is how does the offending server get back in and start sending again. Is there something I need to do as administrator, is it something the sending server needs to restart or perhaps just a time-out period?

That depends exclusively on the sending server and how it reacts to the rejection. Impossible to predict and nothing that you can do about it (other than increase the connection limit). Simon.Simon Butler, Exchange MVP Blog | Exchange Resources

Thanks Simon, we are working with the sending server team to see if they can throttle down their send stream. Dan