I don’t know if it was just me, or if it is a server-wide issue, but aparently, when formail updated at the end of November, it either changed or corrupted my message ID cache. The mail would be sent by exim, to procmail, to formail, and formail would silently drop every message except one junkmail per day.

Not everyone is using a message ID cache. If you are using procmail, you’ll want to check the file ~/.procmailrc and see if you have one. If you don’t see it near the top, then all should be well.