[otrs] Accepting mail only from known senders?

Hi all, My OTRS system (2.0.4 - as found in Debian Stable) is set up to retrieve all messages sent to a specific mailbox (support@xxyy.abc) and autocreate a ticket for each new mail. This is potentially open to abuse. Is there a way to reject (silently or otherwise) any message from an unknown sender, and only create new tickets for messages sent from accounts that OTRS knows about (ie in the "Customer User"'s email field)? Cheers. Grant. :)