Posteo is very aggressive on privacy, reducing exposure, and keeping information safer. So a shorter webmail session timeout may be related to this. Depending on the use case, if this were the only issue with Posteo, the GP could’ve used Firefox with one of the many tab reload or tab refresh extensions to keep the logged in session alive.

Posteo doesn't allow custom domains (a no go) and lies about why they don't.

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.