What's the real reason they don't allow custom domains in your opinion? Why do you say they are lying?

Their support replied with a mumbo-jumbo of techno-babble why it's not possible.

Of course it's possible.

The reason is clear: lock-in.

People are reluctant to change providers if they lose their mail address.

The FAQ (https://posteo.de/en/site/faq) says the do it because a domain has personay identifying information in WHOIS and they would need to store it.

I think for .de domains, you are required to have your personal address in WHOIS if you are not a company.

That's a new claim, and it's also obvious bullshit.

First, I may be okay with it. And why would Posteo store WHOIS data? Unless they want to be a domain reseller, which is not what I asked them about.

Second, there are other TLDs.

I still insist that they do it because of lock-in and that they lie about it.

They wouldn't store whois data of course, but the domain needs to be stored at it can then be used to look it up.

I think mailbox.org supports custom domains and is similar in other respects.

That's a stupid excuse.

They should stop storing the mails themselves, they are full of private information. /s

AFAIK the GDPR explicitly mandates registrar to hide personal information.

Yes. Since May 25, the whois for .de is limited to technical information like DNS server or DNS key.