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.
I toyed with the idea of running my own #Mastodon instance back when I'd first started looking into it. This was also before I knew much about the #Fediverse in general. I decided ultimately to just join mastodon.social and get a feel for things. This I think was the right thing to do.
Since then, I've moved to hackers.town for my microblogging needs, which is a nice community. I've kind of put aside the idea of selfhosting Mastodon, partly because of the resource usage, and partly just because this is working for me. I selfhost a lot of other stuff, like web and email (and this place, which is somewhere between microblogging and normal blogging (macroblogging?)), so at this point I've just been thinking I'd stick with this for a while.
In the mean time, I'm kind of content that at least there are other options out there for selfhosting, in that someone will be able to even if I don't have the urge for it right now.