Microsoft on Thursday announced Hotmail users could block HTML images from appearing in e-mail messages, in a move meant to foil spammers trolling for valid e-mail addresses.

Beginning this week, Hotmail users can choose to block HTML images from appearing in e-mail messages from senders not in their contact lists. E-mail users can open the images in unknown messages after seeing the other contents of the message. The technology targets a common technique used by spammers of inserting "Web beacons" in e-mail messages that verify an e-mail address is valid when a message is opened.

"Spam is no longer just an inconvenience for consumers and the online industry," said Lisa Gurry, MSN's group product manager. "It has become a major problem, one that makes it hard for people to sort through their personal e-mail and reduces productivity."

The move to block Web beacons by blocking HTML images worries some e-mail marketers who believe the moves hinder the effectiveness of HTML messages, which boast response rates nearly double that of text messages, according to DoubleClick. Yahoo! rolled out the option for its users earlier this year.

Microsoft has already taken steps against Web beacons. The beta version of Outlook 11, its hugely popular e-mail client, includes the ability to block HTML graphics from the preview pane.

"If the objective of adding this feature is to reduce spam, it's going to fail," said Al DiGuido, chief executive of e-mail marketing firm Bigfoot Interactive. "We don't think that any feature in or itself will end the [spam] problem."

While spammers often use Web beacons to verify addresses, legitimate e-mail marketers use them as an analytics tool, tracking who opened their mailings.

"There are legitimate uses of Web beacons," said Trevor Hughes, executive director of the Network Advertising Initiative's (NAI) e-mail service provider coalition. "Web beacons are an almost mundane technology."

Last November, the NAI released a set of guidelines for the use of Web beacons, in the hopes of defusing privacy concerns over the use of such tracking technology.

MSN said its move against Web beacons was one of many techniques it has employed in its quest to cut down on the flood of spam. The company inked a deal with leading anti-spam company Brightmail last September to block spam before it reaches customers' in-boxes. In another move, Hotmail added technology that detects automated account signups, in an attempt to stop spammers from using the free e-mail service to send spam.

MSN has also recently joined with AOL and Yahoo! in an initiative to fight the spam scourge and has filed a number of lawsuits against spammers.

MSN said its approach has begun to bear fruit, with the service now blocking 2.4 billion spam messages a day -- a staggering 80 percent of the e-mail it handles. AOL has reported blocking a similar level of spam.