https://support.apple.com/guide/icloud/add-a-custom-domain-m...
Since I am in a time of moving to other city to study on university, I decided to abandon my mail server and migrate to iCloud... so now I am moving every of my service@domain.tld to prefix+service@domain.tld (tagging system that doesn't parse properly on some sites). It's no fun, but at least I'll take off my head caring whether my server is on fire, as it's now Apple's issue.
Do you mind me asking roughly how old you are? I would like to test my theory that email is generational and it's only the over-(insert number here) that still use it as it was originally intended.
I don't have any releases yet, because it's my first that huge app. Also I've never been digging that deep into AppKit framework, so I enjoy writing this, but I also see a lot of downsides where I know that in future I will have to rewrite a few of complex core components of AppKit on my own. It's worth mentioning that app will operate on files (like Pages, Mindnode), not on the internal database, that would be hard to reach for an user that would like to open this file with some other apps (files are SQLite databases for now), like some SQLite manager.