The remote host is missing an update as announced via advisory SSA:2007-215-01.

https://secure1.securityspace.com/smysecure/catid.html?in=SSA:2007-215-01

New Thunderbird packages are available for Slackware 11.0 and 12.0 to fix two possible security issues. This package may also be used on many older versions of Slackware (though we're not certain how far back...) More details about the issues may be found here: http://www.mozilla.org/projects/security/known-vulnerabilities.html#thunderbird