Users sent items in pending status

All users on a certain post office periodically have their sent items get stuck in pending status. I researched this and found the following:

I noticed the Domain where the post office resides which is on our 8.0.3 MTA keeps closing

The post office resides on a SLES 11 box.

I have to go in and stop the post office rcgrpwise stop and restart the post office rcgrpwise start

I have done and rebuild on the post office and MTA, issue is still happening

Any idea on why this keeps happening and how to fix? Thank you

This is the only Post Office on that domain that is having this issue.