I notice that according to the "emails sent" report (visible to staff only), the number of emails sent sharply dropped on October 28, and after that, nearly no emails have been sent. I do not know whether this corresponds to a Discourse version upgrade or not. The weird thing is that the dashboard report (also staff only) reports 988 emails sent today, 0 yesterday, 0 in the last 7 days, 3 in the last 30 days, and 102541 total ever. Perhaps the "today" statistic is somehow transient / computed differently.
The weird thing is that if the issue is only on the receiving side, then Discourse shouldn't know it hasn't been sending mails successfully... right? I am really hoping that today's Discourse upgrade and related fixes will magically resolve this issue.
Edit: I believe the Discourse upgrade indeed changed something for the better. Gleaned from the list of sent emails (staff only), we have:
1 3 hours
1 Nov 3
1 Nov 6
3 Nov 29
6 10 mins
6 52 mins
14 1 hour
150 Oct 27
967 2 hours
The "Oct 27" number is low, since I cut off the output once I got a few (150 it seems ) entries into October.
Those 967 "2 hours" mails are nearly all digest mails too, so it really looks like the system figured out that it hadn't sent any emails in a long while, and has now started doing so again.