[Dovecot] %d in configuration file does not work

I am trying to set up dovecot in a postfix virtual account/domains setting. In the configuration file I would like to use %d in two places, for:

mail_location = /var/mail/vhosts/%d/%n

and to specify the location of the respective passwd file:

passdb passwd-file { args = /etc/auth/%d/shadow }

Looking at the logs, %d is empty while %n works, so dovecot is looking for something in /var/mail/vhosts//myname and /etc/auth//shadow which don't exist. Replacing %d in dovecot.conf with the actual domain name works.

So, the quesiton is, why is %d empty, and from where does it take its value?