mail is the one thing I refuse to self host for the simple reason that despite not being particularly hard to get up and running initially, when it doesn’t work for whatever reason it can be and often is a gigantic pain in the ass to deal with, especially when it’s something out of your control. For personal there’s very good free options, for enterprise those same free options have paid options.
Whether it be gmail having a bad day and blocking you or whatever cloud provider or on prem infrastructure crapping out for long periods of time causing you to be cut off from email for a while and potentially missing incoming mail permanently if the retries time out. Or anything in between. It’s one of those things where I’m glad it isn’t my problem to deal with.
My only involvement with email is ensuring I have a local copy of my inbox synced up every week so if my provider were to ever die I still have all my content.
You might not even be able to install modern OS on it as many are starting to drop support for old hardware, I know the linux kernel did some pruning recently.