Some months ago, the service we had been using for “transactional emails”, aka notifications, discontinued their free tier. This happened at a time we were busy with other things, and I didn’t realize it until much later.
Anyway, there’s a backlog of notifications that the server is still sending out. It will probably take a couple days to catch up because there are inbuilt limits on the server which limit the number of messages it sends per hour (to prevent spam attacks).
So I’m making this as fixed for now. Over the years, email services have used a free tier to get customers, then cancelled the free tier after they grew to a comfortable size. They like to say it’s free forever, which means something different. I mean, it’s free, so it’s hard to complain, but I wouldn’t be surprised if we go through something similar (4th time?) at some point.