Have you looked into Purely mail? This is what I use for my custom email needs. I don’t remember all the pros and cons, but the big one that scares most people off is it’s run by one guy. So if something happens to him, you’re potentially SOL. You could probably migrate to a new service, but could potentially be a huge pain.

I volunteered as tribute to be one of these ‘Friends’

Not OP, but I run both because Plex was/is easier to setup non-techy family members and easier to get on their TVs/devices.

Plex has been pissing me off though and I’d like to move to Jellyfin, but sounds like a PITA to transfer everything over. Gonna have to get clever with that probably