TL;DR: - NO TL;DR: this time, I spent hours writing, you should spend minutes reading. - OK... I explain in WAY TOO MUCH details how to setup a mail server EDIT (2019-10-26) OpenSMTPD 6.6.0 was released today, the article was updated to reflect that it applies to the current release and no longer a development version.
EDIT (2019-12-15) I have refactored the article to remove the political aspect behind self-hosting, which is now a standalone article, as well as a blurb on reputation since this will become part of a serie of articles.