Before today, mailbox.org’s 2FA mechanism was unorthodox. In the login screen, you typed in the TOTP in the password field and then added a 4 digit static pin at the end. This got people confused, as it’s different than the usual login+password then TOTP. Now it’s just like that.
There’s also other goodies, like separate passwords for IMAP and SMTP, WebDAV, CardDAV/CalDAV (one password for both), Exchange Sync. Before today, you’d be using your main mailbox.org password for all of the above. Looks like IMAP access is not even possible without creating a separate password https://kb.mailbox.org/en/private/account-article/how-to-use-two-factor-authentication-2fa/
There doesn’t seem to be support for the YubiKey TOTP anymore. No passkeys or hardware webauthn either for now.
mailbox.org is based on OpenXchange.
Bit of trivia but I think I know why the 4 digit pin thing existed! It’s an out-of-the-box feature on freeRADIUS, I ran across it in a pfsense environment in the past. I thought it was neat (esp. in the absence of passwords, this was primary auth with public keys and then 2fa on top) but ultimately too convoluted for most users