The remote host is missing an update to nullmailer announced via advisory DSA 198-1.

https://secure1.securityspace.com/smysecure/catid.html?in=DSA%20198-1

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.