I threw together a toy mail server on an OpenBSD machine I have. Just an OpenSMTPd setup for local users, with mutt. It works, although of course deliverability will be terrible. My Gmail account accepted a message from it but marked it as spam. But, when I un-spammed it and replied, it kept the thread in my Inbox.
So, most people would consider it useless. I'd consider it slightly not-useless. Not usable the way we typically desire email, but could come in handy, if only as a teaching tool.
I've thought before about treating email kind of like the #fediverse, as in deciding to set up a server just to talk to specific other servers, instead of trying to reach any mail server. In other words, using it just between friends. I would like to put together a little guide for this sometime. Could be fun/interesting/slightly not-useless.
Find me at @firstname.lastname@example.org