I tend to keep things simple so if I can it’s easier to not set up the separate auth middleware when there’s already an AD comparable system in place.

Another option I’ve used before is called Neth Server, but that’s more one of those SOHO all-in-one systems rather than a dedicated mail box.

https://community.nethserver.org/

Just beat me to it…

The one thing that they don’t have yet last I updated, though they’ve been working on it for a while, is a prod ready LDAP/SSO connection. I had the dev branch working with Keycloak, but never got plain LDAP to function.