Here’s what the spammer did to pull off this little magic trick:
1. The spammer devised a query string which yielded only his or her URL as result of an advanced Google search.
2. The spammer then simulated the click of the "I'm Feeling Lucky" button (notice the '&btnl=' at the end of the above URL) that will take you to the URL of the first result that comes up for the entered search query.
Example of manipulating Google's "I feel lucky" search:
3. Lastly, the spammer packed this URL into a regular email and sent it out to evade spam filters.