I still run my own. It's opensmtpd based. It use to be postfix so it's accumulated some tech debt over the years, but it still works really well:

https://battlepenguin.com/tech/upgrading-opensmtpd-6.3-and-running-e-mail-in-docker/

but yea, it's not easy, but probably worth learning for young and upcoming sys admins. There's a lot of history there, buried in the SMTP protocol and all the things that have been bolted onto it over the years.