The remote host is missing an update to courier announced via advisory DSA 247-1.

https://secure1.securityspace.com/smysecure/catid.html?in=DSA%20247-1

The developers of courier, an integrated user side mail server, discovered a problem in the PostgreSQL auth module. Not all potentially malicious characters were sanitized before the username was passed to the PostgreSQL engine. An attacker could inject arbitrary SQL commands and queries exploiting this vulnerability. The MySQL auth module is not affected. For the stable distribution (woody) this problem has been fixed in version 0.37.3-3.3. The old stable distribution (potato) does not contain courier packages. For the unstable distribution (sid) this problem has been fixed in version 0.40.2-3. We recommend that you upgrade your courier-authpostgresql package.