Just don’t tell us how many nines you have, hey?
loading story #50483321
It is not difficult to keep a mail server online.
FreeBSD(ZFS+jails old but gold) + postfix + dovecot.
Ha nearly the same setup here, FreeBSD/ZFS/opensmtp/dovecot/rspamd
And yes uptime (with short outages(updates etc)) is not that important for a mail-server, since the sender re-transfers in intervals (mine is 5min the 8 then 15 then 60 then 180 then something up to 2days)
So the uptime for the service is 100% (maybe 99.999) over the last 8 years.
Mail servers are easy, getting the other mail servers (Microsoft,Gmail,etc) to play nice with yours is harder.
Not that hard, for example thats a good tester:
and
https://mecsa.jrc.ec.europa.eu/ (offline atm Azure??)
loading story #50482406
infinitely many, so far...