The remote host is missing an update as announced via advisory SSA:2008-366-01.

https://secure1.securityspace.com/smysecure/catid.html?in=SSA:2008-366-01

New mozilla-thunderbird packages are available for Slackware 10.2, 11.0, 12.0, 12.1, 12.2, and -current to fix security issues. More details about the issues may be found on the Mozilla site: http://www.mozilla.org/security/known-vulnerabilities/thunderbird20.html