When antispam activist Mark Mumma received unsolicited emails advertising cruise vacations two years ago, he posted a report on his Web site and threatened to sue Omega World Travel. But Mumma met with an unpleasant surprise: He was the one sued in federal court by Omega World Travel and its subsidiary Cruise.com, which demanded $3.8 million in damages for defamation. The 4th Circuit Court of Appeals <A HREF="http://news.zdnet.com/2100-9588_22-6138874.html?tag=nl.r23660">sided with the alleged spammers.</A> In a little-noticed opinion issued in mid-November, a three-judge panel acknowledged the email messages in question may have included a false Internet address and a nonworking "From:" address, but concluded that they nevertheless were permitted under the federal antispam law known as the Can-Spam Act.