Our incoming mail servers got very clogged up.  There were a number of contributing factors.  When I recently moved them I didn’t allocate enough RAM for them to operate efficiently.  CentOS recently pushed out a broken update for fail2ban and so a lot of crap that is normally blocked was hitting them.  A virus was released in the wild and circulated fairly well before the clam-av folks got a signature for it in their database.  There was no bounce time limit configured.

     All of these things have been addressed and now they’re cleared out and functioning normally.