[Dovecot] Domain part / auth_username_format

[Robert Coup]

Hey folks,

I want to authenticate a pop3 user without a domain part and then use 
the domain part to specify a Maildir subfolder to grab messages from.

So, this was my attempt:
  # use the domain part to access a subfolder
  mail_location = maildir:/mail/%n/Maildir/%d
  # strip of the domain part for authentication purposes
  auth_username_format = %n

login: testuser at .INBOX.SubFolder

Authentication of testuser works great, but mail_debug shows me that the 
domain-part (%d) is empty when expanding mail_location.

Any bright ideas?

Rob :)

[ Dovecot 1.0.rc15, Debian Sarge, PAM auth. ]