please keep in mind, that both providers (posteo-de and mailbox-org) are political active (more or less leftwing / left).
posteo supports "freiheit statt angst". in this alliance you find radical left-wing groups, left-terror-supporting groups, radcial left-green groups e.g.
mailbox-org have a strange policy for users they want to use their service (or not). mailbox-org parent company behind (jpberlin-de / heinlein-support-de) calls themselves a political (left-wing) provider. jpberlin tries to do a check of the political convication and if you fail (right thoughts are not allowed) then you can not use / your are not allowed to use jpberlin/heinlein-support/mailbox-org service. they never ever described how they check the political convications of their users but here they write about "poltical provider" and what they mean with it (only in german)
Code:
https://anonym.to/?https://www.jpberlin.de/die-jpberlin/wieso-politischer-provider/
before you sign up one of this providers you should now this background information. i give you only the facts, the evaluation you have to do by your own.