[otrs] Postmaster filtering not working

Hi, I'm trying to setup some postmaster filters, but they don't seem to be working. I'm retrieving mail from an IMAP box using PostMasterMailbox.pl. I've created a rule named "Alerts" which is: Match: To = EMAILADDRESS:alerts@example.com Set: X-OTRS-Queue -> Alerts However, I'm not seeing any messages getting delivered to the "Alerts" queue. "Alerts" is a Sub-Queue of "Monitoring" - does that make any difference to how I specify the queue name in the filter? i.e. do I have to specifiy the parent queue too, e.g. Monitoring:Alerts, or similar? Any other ideas why this is not working? Thanks, R.