[Dovecot] User login for SMTP but not for IMAP/POP?

Hello everybody,

I have a question, though I'm not sure if it's a matter of dovecot or postfix. Or if it's even possible to do at all.

If I understand it correctly, SMTP authentification is done via SASL. When a user wants to login Postfix queries an external user database (dovecot). Therefore the SMTP-users are identical to the IMAP/POP-users.

I want to achieve the following: I want some user credentials (username&password) for a user that is able to login via SMTP, but who doesn't have a mailbox and therefore shouldn't be able to login via IMAP/POP.

Is this even possible? How can it be achieved? Or: does this behaviour have a special name, which I can google for?

Regards, Martin