The root of the problem was an upstream deficiency in wget. The dumb forums use a cron job that does a wget call in order to trigger mail notifications. The wget that is on board is using some library that doesn't have modern root CAs and so it was failing to process the URL because of the https mistrust. This is why some mail functions were working but notifications were not.

The forum software is dumb. Sorry for the long delay in getting it diagnosed and fixed. It was tricky to find.