You've received a postcard from an unidentified neighbor/colleague/friend/etc, etc but DO NOT CLICK ON THE LINK IN THE EMAIL. The link brings the browser to a website that attempts to load scripts to make your computer part of a botnet which hackers can control to do things like sending SPAM or hosting malware servers.
More technical details found @
http://www.ericconrad.com/http://isc.sans.org/diary.html?storyid=3063