A little while back i saw a program that would generate a reply mail that looked identical to a undeliverable mail : user doesnt exist situation.
A lot of the spam systems will remove that email address from the database if they recieve a returned mail saying that it doesnt exist.
Seemed like a decent bit of software......but i never grabbed it....nor can i remember the name anymore
Ahh well....back to the blocked sender and turned off preview pane method again