yeah, while reading this i was going “What maniac exposes SSH to WAN anyway…?” the whole time. I fail to see any reason or point that can be justified exposing SSH to WAN.

Posteo does not support custom Domains, because it’s unsafe and inherently complicated to use with all the security settings of a modern email system turned on

Depending what you need and want, I can highly recommend posteo.de and while they are not as polished and use standard DAV, it works absolutely flawless, they are relatively cheap, based in German and have certifications for all kinds of security.

If you want something like Google Workplace, get a Nextcloud. Still not as polished, but open and if you want to change the provider, you can just do so. I recommend a combination of Nextcloud and Posteo.

And if you need a custom domain with everything bundled together, I can recommend xaweho.de. though they are not cheap…