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.
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.
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.
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.
I can second this impression on a personal level and for two small business.