[Dovecot] lmtp bouncing -- Invalid parameters (in reply to MAIL FROM command)

I just configured postfix' virtual_transport to point at my dovecot director, but am seeing occational problems like this:

Aug 31 11:50:06 smtpgw postfix/lmtp[5339]: 69E2F5410D: to=<email@example.net>, relay=loadbalancers.example.net[192.168.42.17]:24, delay=0.15, delays=0.14/0.01/0/0, dsn=5.5.4, status=bounced (host loadbalancers.example.net[192.168.42.17] said: 501 5.5.4 Invalid parameters (in reply to MAIL FROM command))

but can't find anything interesting in the dovecot logs.. Anybody have ideas for how to make postfix play nice with dovecot's lmtp ?

smtpgws% rpm -q postfix
postfix-2.3.3-2.1.el5_2

loadbalancers% rpm -q dovecot
dovecot-2.0.13-2

backendmailstorers% rpm -q dovecot
dovecot-2.0.13-2

-jf