I did some looking through announcements and phab tasks to see if I could find any reported maintenance or outage that might indicate that we didn't get the message to the smtp relay and came up empty. The closest thing I could find was a dns change for cloud vps instances in september, but if that were a problem, all mail delivery would have been impacted.

We did have a per-user-account-scope issue pop up in october, which is when unconfirmed email addresses were removed from oauth responses:
https://phabricator.wikimedia.org/T283456

It looks like we're checking to see if we have a non-empty email address before trying to send, so I don't think that's it either:
https://github.com/WikipediaLibrary/TWLight/blob/b4df82f92b9abe81d94b8d400954f192c19dd1e2/TWLight/applications/management/commands/send_coordinator_reminders.py#L77-L84

Email receipt depends on a whole lot of things going right across multiple organizations and networks; if it looks like it's working moving forward from today, I think we'll just file it under "unknown ephemeral issue outside of our control"

Marking as stalled until we hear back about that.