[Dovecot] dovecot delivery log

[jalal]

Trying to debug a delivery problem recently (with Postfix) and I
realized that Dovecot wasn't logging any of the delivery information.
I'm using Postfix to recieve mail, which is passing it on to Dovecot's
deliver to actually deliver the emails into the mail boxes.

In dovecot.conf, I have:
protocol lda {
  log_path = /var/tmp/dovecot-deliver.log
}

[root]# dir /var/tmp/
total 2
drwxrwxrwt  2 root     root     1024 Oct 22 12:30 .
drwxr-xr-x 24 root     root     1024 Aug  5 08:03 ..
-rw-------  1 vmail    vmail       0 Oct 22 12:11 dovecot-deliver.log

The log file gets created OK, but then stays at zero size.

Using:
Postfix 2.3.3
Dovecot 1.1.4 (and previously 1.0.9)

Any clues as to how I can have a delivery log?