But you cannot have your own domain on posteo as far as i know.

GDPR clearly says, if there is a valid reason for storing your data, they can store it (no timelimit). Like you can store data for invoices etc for 10 years too even when you ask them for deleting your data.

Iguesseverybody also agreed to it when you registered.

I do not see any valid reason why they would delete acvounts, like saving 1 line in a database?