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??)
Getting a clean ip is hard. I have my personal email server setup and had to get it off of some black lists maintained by baracuda. Although i agree personal self hosted email is fairly straightforward and ideally more people should do it. Hetzner provides cheap and reliable vms.
Reliable is a flexible word. Hetzner does frequent, albeit often brief, maintenance operations that would incur a down time on a high-throughput live service. Not impossible to design around with enough redundancies, but far from "I have one FreeBSD box that's been up for 467 days with 100% uptime" that people used to brag about in the early 2000s. I have few VPSs and bare metal instances that I just `sudo reboot` when there is a kernel update with practically "zero" impact or downtime for myself. Something unimaginable to do in a production system where you need to cordon, traffic drain, update, reboot, uncordon then orchestrate that across an entire fleet of instances with different roles and characteristics in a specific order.
>Hetzner provides cheap and reliable vms.
True, i have some hetzner (not vm's but metal), and Vultr (just vm's), i just had one bad IP until now (i just changed it instead of trying to get it removed from a blacklist)