Apparently, this is what happens when spammers use your domain for sending out spam, and receiving domains don't, at a minimum, check the SPF records (looking at you,
root@mail:/var/mail# postsuper -d ALL deferred
postsuper: Deleted: 114953 messages
Of course, what I have learned with this, and the 7G of mail log files, is that I need better monitoring of my mail servers. That these 114953 emails had 4 days of delivery attempts and I didn't notice is, well, really not a good thing.