Just for information, in case anybody else --like me-- runs into this issue, it is bug 8146, fixed as of 2008-02-21 (which probably puts it in version 5.03).
However, if you can't upgrade for some reason -- again like me-- here is a work-around that makes it persistant so you don't have to opening up a console and running zmprov to reset the settings all the time.
Edit the file /opt/zimbra/conf/postfix_recipient_restrictions.cf
and change (don't comment it out, actually change it) each line that has an RBL you want to use. For example, from this:
%%contains VAR:zimbraMtaRestriction reject_rbl_client cbl.abuseat.org%%
Reload your postfix settings, by making a change in the GUI, via "Global Settings", "Anti-Virus" tab and changing from 2 hours to 5 hours for example, or zmcontrol stop & start.
The 'zmprov gcf zimbraMtaRestriction' command will not show it taking effect, but if you watch the appropriate log you should see it start rejecting messages. 'tail -f /var/log/zimbra.log | grep NOQUEUE' works on my version, other versions may log it to different places.