SAN FRANCISCO (09/22/2003) - The Web-based e-mail service Mailblocks Inc. can effectively block junk messages by using challenge/response technology that forces senders to prove that they're real. In the past, however, people whose addresses had been verified by one Mailblocks user would be challenged anew each time another friend or colleague began using Mailblocks. The updated service keeps track of all verified users, eliminating repeat challenges. Other improvements include customizable challenges and a very much needed spelling checker. The yearly fee remains US$10.