Dovecot with pam authentication and user@domain

Hello all, I'm trying to setup dovecot for local users with pam authentication. The passdb and userdb entries are as follows:

passdb { args = username_format=%n driver = pam } userdb { args = username_format=%n driver = passwd }

Using "doveadm user chriss" returns the user record as expected, but "doveadm user chriss@testmail.ch-sc.de" gives an error: field valueuserdb lookup: user chriss@testmail.ch-sc.de doesn't exist

As far as I understand, username_format=%n should drop the domain part and only search for the user in the userdb, but it doesn't. What am I missing?

Greetings Christian