I have no idea why they disallow those. But you could email them and ask. They usually respond in a day or two. Side note: Posteo does not allow own domains (y

I have no idea why they disallow those. But you could email them and ask. They usually respond in a day or two. Side note: Posteo does not allow own domains (you can only choose from the list of Posteo.* domains that the company owns). The reason for that is mentioned in their FAQ (in short, the company doesn’t want to have or store customer identifying information wherever possible).