All MTAs have retries baked in, so running a self-hosted email server that receives mail is actually one of the most forgiving services in this respect. If your server is offline, the sending server will retry several times over 24-48 hours before it gives up. Even the big cloud email providers will do this.

That said, there are other aspects of running an email server that require some extra rigour, but they’re more on the sending side (making sure emails you send to other people actually land in their inbox). Doable, but one of the more challenging things to self-host.

OpenProject is what you’re looking for.

I’ve tried dozens of clients and Arpegi is my favourite. I’ve been using it for a year.

Arpeggi on iOS is the best I’ve found. Available via TestFlight invite only. I plan to switch to Android in the next year, so I’m always interested to hear what clients people like over there too.

I use Navidrome and highly recommend it. Nice Canadian developer.

I also use GitLab.

One option is for you to mirror Navidrome on GitLab. I will happily use your mirror instead of GitHub.

Another option is to reach out to the Navidrome dev and propose moving to GitLab. If you open an issue to that effect, I would add a comment with my support.