The remote host is missing an update to courier announced via advisory DSA 784-1. A problem has been discovered in the Courier Mail Server. DNS failures were not handled properly when looking up Sender Policy Framework (SPF) records, which could allow attackers to cause memory corruption. The default configuration on Debian has SPF checking disabled, so most machines are not vulnerable. This is explained in the courier manpage, section SENDER POLICY FRAMEWORK KEYWORDS. The old stable distribution (woody) is not affected by this problem.

For the stable distribution (sarge) this problem has been fixed in version 0.47-4sarge1. For the unstable distribution (sid) this problem has been fixed in version 0.47-6. We recommend that you upgrade your courier-mta package. https://secure1.securityspace.com/smysecure/catid.html?in=DSA%20784-1