@jwildeboer @blaisealleyne @knowprose @homelab A close friend and I have been running a mail+ server out of my basement for over 25 years, starting off on a 1U Sun Netra. The basement, the ISP, the hardware, and the daemons have changed, but it's had the same core for over a decade (exim4, dovecot, spamassassin, prosody, nginx, radicale, rsync backups).
Over the years, our most persistent headache has been poor ISP netblock reputation. For a while we had a separate outbound server, but our current static IP has been blessedly free of RBL bans, so Outlook, Gmail etc accept our messages.
We have custom exim rules to provide per-user local-part delivery filters and sender-callout/greylist whitelists, which makes on-the-fly exceptions easier, but also caused my friend to work hard to rewrite the exim4 config when an update broke it. The dovecot config migration was a lesser pain by comparison.
I would not be able to maintain this system safely without a partner who has more time than I do to attend to "bursty" maintenance problems and mitigate security vulnerabilities. It also helps to have only a half-dozen users! So for now, we keep on trucking...