The remote host is missing an update to dkim-milter announced via advisory DSA 1728-1.

https://secure1.securityspace.com/smysecure/catid.html?in=DSA%201728-1

It was discovered that dkim-milter, an implementation of the DomainKeys Identified Mail protocol, may crash during DKIM verification if it encounters a specially-crafted or revoked public key record in DNS. The old stable distribution (etch) does not contain dkim-milter packages. For the stable distribution (lenny), this problem has been fixed in version 2.6.0.dfsg-1+lenny1. For the unstable distribution (sid), this problem has been fixed in version 2.6.0.dfsg-2. We recommend that you upgrade your dkim-milter packages.