[Dovecot] logging IMAP username rather than UNIX username

Hi there

I'd like to change my setup to log the _actual_ IMAP user's login, rather than the UNIX username. My system uses a virtual uid setup with qmail, where the mailboxes are owned by the 'vmail' user and authentication is via checkpassword using user%domain (ie email, just with % instead of @). So dovecot is recording _all_ logins as:

imap-login: Login: user=<vmail>, method=PLAIN, rip=127.0.0.1,

lip=127.0.0.1, secured

I have the following log format specified:

login_log_format_elements = user=<%u> method=%m rip=%r lip=%l %c

I have also tried:

login_log_format_elements = user=<%n@%d> method=%m rip=%r lip=%l %c

However, this results in a log file entry of:

imap-login: Login: user=<vmail@>, method=PLAIN, rip=127.0.0.1,

lip=127.0.0.1, secured

How do I configure dovecot to log the _actual_ username which the client supplies during authentication?

Thanks Dale