postfix + dovecot LDA: bounce, user unknown

[cbrace]

Greetings fellow former BSDF denizens,

I recently set up Dovecot to work in conjunction with Postfix as a IMAP(s) server on my FreeBSD 7 box. It works fine.

Now, I would like to configure Dovecot as the LDA, so as to be able to use the sieve plugin for sorting mail on the server, but whenever I activate Dovecot LDA in Postfix's main.cf and send myself a test messsage, I get an error message in maillog:

May 8 20:47:17 venus postfix/pipe[36077]: 4022B5C38: to=<cb@lim.nl>, orig_to=<colin@lim.nl>, relay=dovecot, delay=0.02, delays=0.01/0/0/0.01, dsn=5.1.1, status=bounced (user unknown)

Postfix's virtual LDA has no such problem:

May 8 20:49:39 venus postfix/virtual[36181]: F3DF05C37: to=<cb@lim.nl>, relay=virtual, delay=0.05, delays=0.03/0/0/0.01, dsn=2.0.0, status=sent (delivered to maildir) 
May 8 20:49:39 venus postfix/qmgr[36091]: F3DF05C37: removed

Here is the LDA section in dovecot.conf

postmaster_address = postmaster@lim.nl
mail_plugin_dir = /usr/local/lib/dovecot/lda 
mail_plugins = cmusieve log_path = /var/log/dovecot-deliver 
info_log_path = /var/log/dovecot-deliver 
auth_socket_path = /var/run/dovecot/auth-master }

My maildirs are here:

mail_location = maildir:/home/colin/vmail/%u

and Dovecot IMAP has no problem retrieving messages from that location. What could be going wrong with Dovecot's delivery?

Thanks, and have a nice day.