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.
Maybe most people using Mailbox know about this but I’ll still mention that using Mailbox kinda requires having your own domain.
Reason is the same as Posteo (unless Posteo changed something lately) : mail adresses will get recycled after some time when you stop using the service and close your account.
Most other providers blacklist adresses so they can’t get reused when an account gets deleted.