This project's scope is rather impressive, and from what I've gleaned from skimming its docs, it might be the perfect solution for people who want to self-host email.
It tries to have most of the advanced/difficult things (like getting DKIM set up right) taken care of for the postmaster, as far as that is possible within a single application. I love that it will print the DNS records you'll need to set up in your zone during setup.
If I were to start over my personal email setup (postfix with postscreen, dovecot with fts, opendkim, amavis/spamassassin), I sure would try it with this.
I set up docker-mailserver[0] Monday in ~6 hours, most of which were me trying to be fancy using podman instead of docker and dealing with SELinux. But then again I did choose it over mail-in-a-box for just that level of customization. Obviously I can't tell how reliable it will be in the long run yet, but since it's using the classic Postfix/Dovecot stack I expect it'll be pretty stable