NEW YORK (CNN/Money) - The Federal Trade Commission plans to require that pornographic e-mail spam carry explicit messages in what amounts to an electronic version of the plain brown wrapper, a person familar with the plan said.

The commission will issue a new rule ordering marketers issuing porn spam to put the words "SEXUALLY EXPLICIT" in the subject line of any e-mail with raunchy content, the source told CNN.

Supporters said including those words will help e-mail filters keep spam away.

But critics said the rule may be ineffective and unenforceable. It may also be challenged as unconstitutional.