yep, you are right - custom domains are not supported by posteo. But mailbox.org supports them afaik

I relays all my mails from my custom domain to my posteo mailbox for the last 10 years. No issues there. (I do this via a minimal postfix on the VPS on that domain). The downside is, that I can only receive mail on that domain, but not send any.

They all get the job done. Posteo has no free plan and only one paid plan which is the cheapest of the three (1€/mo).

Tuta is nice, but you have to use their app or website to read your mail.
Search is slow and often requires you to rebuild the search cache.
And last time I used Tuta, I couldn't sort my email in other ways than time.
It's very secure though and allows for E2E encryption with passwords.

Posteo allows you to use any IMAP client to access your mail.

I use posteo for mail/calendar and proton for everything else.

With the convenience of auto distributing your nudes on your behalf.