The remote host is missing an update to horde3 announced via advisory DSA 1642-1.

https://secure1.securityspace.com/smysecure/catid.html?in=DSA%201642-1

Will Drewry discovered that the Horde, allows remote attackers to send an email with a crafted MIME attachment filename attribute to perform cross site scripting. For the stable distribution (etch), this problem has been fixed in version 3.1.3-4etch4. For the testing distribution (lenny), this problem has been fixed in version 3.2.1+debian0-2+lenny1. For the unstable distribution (sid), this problem will be fixed soon. We recommend that you upgrade your horde3 package.