The remote host is missing an update to dovecot announced via advisory DSA 2252-1.

https://secure1.securityspace.com/smysecure/catid.html?in=DSA%202252-1

It was discovered that the message header parser in the Dovecot mail server parsed NUL characters incorrectly, which could lead to denial of service through malformed mail headers. The oldstable distribution (lenny) is not affected. For the stable distribution (squeeze), this problem has been fixed in version 1.2.15-7. For the unstable distribution (sid), this problem has been fixed in version 2.0.13-1. We recommend that you upgrade your dovecot packages.