The remote host is missing an update as announced via advisory SSA:2006-120-01.

https://secure1.securityspace.com/smysecure/catid.html?in=SSA:2006-120-01

New Thunderbird packages are available for Slackware 10.2 and -current to fix security issues. More details about the issues may be found here: http://www.mozilla.org/projects/security/known-vulnerabilities.html#thunderbird