A problem has been discovered in nullmailer, a simple relay-only mail transport agent for hosts that relay mail to a fixed set of smart relays. When a mail is to be delivered locally to a user that doesn't exist, nullmailer tries to deliver it, discovers a user unknown error and stops delivering. Unfortunately, it stops delivering entirely, not only this mail. Hence, it's very easy to craft a denial of service.
This problem has been fixed in version 1.00RC5-16.1woody2 for the current stable distribution (woody) and in version 1.00RC5-17 for the unstable distribution (sid). The old stable distribution (potato) does not contain a nullmailer package.
We recommend that you upgrade your nullmailer package.