We have a backup mail server that relays mail to primary servers for about 150 domains.  We are using ORFilter on both the primary and backup mail servers. The backup mail server is just running IIS's SMTP service and does not know about all the users on the primary server.  The primary servers are using recipient filtering in Exchange.

The problem we have is the backup mail server getting swamped with NDR's as recipients are denied on the primary server and end up on the backup mail server.

I'd like to know if ORFilter can be used to filter outbound mail...that is, I could create a keyword filter for these NDR's related to invalid senders and choose AbortDelivery by setting the score appropriately.  Does anyone know if this is possible with ORFilter?  And if so, is it just configuring the keyword parameters appropriately?