I’ve used both in the past but am now using neither. If being able to use a custom domain is a requirement then you really only have one option between these two: mailbox.org.

Neither of them truly “win” though and I wish people would stop it with these reductive questions.

Used posteo for 4 years. Pretty good, rules engine is better than a lot of other ones. Support also answers fast.

A few time-out or slow moments probably because they have too many clients. Visible only during a minor part of my 4 years.

But some people didn’t receive my mail, which made me change mail service.

Don’t forget Tuta and ProtonMail.

I haven’t tried Posteo, but I can vouch for Mailbox being absolutely amazing. I’ve been with them for years and picked them at the start of my degoogling journey. I was really happy to have a single service that took so much of google off my phone…email, contacts sync, calendar, online drive, etc etc.

Personally, I prefer Posteo. It’s cheap, it’s simple, the pricing is flat, and it’s fully open-source (and LibreJS compliant).

Eager to return to this thread when more people have had their say.

I used mailbox.org for some time. They gradually removed features from their “basic” 1 euro plan until it was the same as posteo. So I moved to posteo.