The new rules provide that marketers advertising in one joint e-mail can designate which company will be considered the sender and will be responsible for Can-Spam compliance. That designated company will have to make sure that the e-mail gives recipients a way to opt-out of future ads, and also will have to put its name in the "from" line and have a valid postal address in the e-mail. The multi-sender scenario often occurs with travel marketing, where airlines, hotels and car rental companies send one combined message to e-mail recipients.

Janis Kestenbaum, an FTC lawyer in the Bureau of Consumer Protection, said the new rule aims to allow marketers who want to use joint e-mails to easily to do so, while also providing crucial information to recipients. "We were looking to help consumers to readily identify who the quote-unquote 'sender' is within the meaning of the Can-Spam act," she said.

The final rule marks a change from the FTC's 2005 proposal, which said that any company that controlled the content of an email would be considered a "sender." But the commission decided that standard wasn't workable because all marketers typically have some input into the ad copy, Kestenbaum said.

The FTC also will continue giving marketers up to 10 days to stop sending messages to consumers who opt-out. The 2005 proposals would have curtailed the time to three days, but the agency decided that change wasn't necessary.

"There wasn't any evidence that people were using that 10-day period to mail bomb people, so it wouldn't help consumers to shorten it," Kestenbaum said.

The new guidelines also clarify that marketers may not require recipients to pay a fee or complete a survey, or provide other extraneous information when opting out of future ads.

When the FTC proposed the rules three years ago, more than 150 organizations, companies and advocates weighed in with comments, including the Online Publishers Association, Time Warner and the Electronic Privacy Information Center. The new regulations will become effective 45 days after they are published in the Federal Register, which is expected to occur later this month.