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.