I tried to find out if you can use Posteo with a custom domain. And the short answer is: no.
Andy Schwartzmeyer found out the long answer. 👏😀 I implemented it for my blog and so far I’m happy. Interestingly enough it’s the receiving side that needs an external service. I currently use Forward Email‘s free tier, because I already have publicly known email addresses that I automatically collect emails from.
I always had the impression that sending mails is made more difficult in order to combat spammers, but what do I know. 🤷 Sending only needs to include Posteo in the SPF rules and adding a sender identity (usable after a one week cool down 🙈).