You got me really interested here, I ran my own mailserver years ago and eventually just gave it up. I am getting rid of Google Workspace and have been planning a migration to Proton for two domains. But this sounds like a fun project. Any advice? I am going to check out Mox and Stalwart.
What providers are good hosting candidates, I have a website on DO, but from my understanding their entire ranges are blacklisted heavily.
If I remember rightly DO have some restrictions like port 25 on ipv6 outbound being blocked.
I can't speak for all of them but I use mythic beasts in the UK for one mail server (they are a very knowledgeable old school host) and it has been good. I also have dedicated with OVH which is fine, and a couple small scale (eg simplelogin, a notification server) with IONOS but they only deliver to me so I can't say how reliably they deliver elsewhere.
Mox is great but I think it's still alpha. I've been using it for 2 years in production for a small traffic domain. The other I use Exim (with mythic beast's Sympl that sets it up) but it's a little more hands on at the beginning
Excellent thanks