Sexually explicit Internet spam (search) must now carry a warning label. A Federal Trade Commission (search) rule went into effect Wednesday requiring that unsolicited commercial e-mail that contains sexually oriented material include the words "SEXUALLY EXPLICIT" in the subject line.

The rule also bars graphic images from appearing in the opening body of the message. Instead, the recipient must take some action in order to see the objectionable material, either by scrolling down in the e-mail or by clicking on a provided link.

Jonathan Kraden, staff attorney with the FTC's Bureau of Consumer Protection (search), said the label "should help the computers to filter if a computer user decides to set their filtering system up to recognize these two words."

He added, "It should also help the e-mail recipient filter visually so they can go through their mail and decide which messages they want to see."

Industry advocates are critical of the rule, which is part of the federal "can spam" legislation that went into effect in January.

"This is a back door effort to violate people's first amendment rights, whether well intentioned or not," said attorney Jonathan L. Katz of Marks & Katz, LLC, in Silver Spring, Md. He is a first amendment lawyer whose clients include members of the adult entertainment industry.

Spammers who violate the rule face possible imprisonment and criminal fines of up to $250,000 for individuals and $500,000 for an organization. But tracking down violators can be difficult because spammers often try to escape being directly identified by using forged return addresses or by bouncing their e-mails through unprotected relay computers on the Internet.