Email notification not working anymore?

The problem arose when I switched hosting, and the forum ran under PHP7 for a bit. Bad plan - MyBB isn't so happy with it, and a number of things have been failing behind the scenes, without error logs ... "only" warnings, but with tables not properly updated or ever so slightly corrupted (eg thread subscriptions). I managed to plug some of the leaks, as it were, and am going through a painful table by table comparison, fixing things, so that we can run the latest version of MyBB, which claims PHP7 compatibility.

MyBB has a version upgrade/migration tool, which hasn't been helpful. Running for 3 or 20 hours reaches the same point, which is why I chose to stick with the earlier codebase, not realizing PHP7 had been enabled for the forum.

The code is a can of worms, to be polite (not our doing), and I look forward to one day having our own elegant & simple forum built with CI. In the meantime, I am chipping away at this as quickly and carefully as I can. This should be a onetime pain, as we have total control over the new server.

Apologies for the issues, and thank you for your patience!