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.