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