To the sysadmin population of the Fediverse: do people have any numbers on how long common mail senders will retry sending mail if your MX is unreachable? Once upon a time people retried for many days, but my impression is that quite a few places now stop trying and bounce the email after quite short intervals, like a day.
(Boosts and practical experiences welcome, like "my MX was down for three days and I still got all that email sent from GMail".)
Before the days of preprocessors (i.e., Spam Assassin, Barracuda, etc.), notifications were sent to the catch-all admin account once every 24 hours, then 5 cycles(?) of notifications triggered it being marked as a 'dead letter', and dumped into a to-be-deleted later holding directory. I think those were generally held for 30 days. Server admins could retrieve and reroute them manually.
After preprocessors, things changed and all bets were off on expecting default timings.
Chris Siebenmann
LumiWorx