Microsoft has added new spam-fighting tools to its MSN Hotmail service in a move that it says underscores its multi-tiered approach to eliminating spam.

The new tools come as email service providers such as MSN and rivals Yahoo and AOL race to diminish users' spam fury, which has heightened in the face of a seemingly endless tide of unsolicited commercial messages.

Demonstrating the extent of the problem, Microsoft said that it blocks roughly 2.4 billion spam emails a day, or nearly 80 percent of messages that reach MSN servers, via layers of filtering and with the help of several spam-fighting utilities.

A new tool that was introduced as part of an upgrade to the Hotmail service this week is a technology that prevents images in email from loading unless the email sender is listed in the receiver's contact list. Aside from clogging up users' networks, Microsoft said that many of the images in spam messages contain 'Web beacons' which alert the sender when the message is opened, allowing spammers to distinguish active email accounts.

Users can still choose to scan the text and manually open the images if they wish.

Another recent antispam addition is what Microsoft calls a Human Interactive Proof technology, which requires customers to interpret and manually retype a random sequence of letters and numbers in an attempt to keep spammers from setting up computer-generated accounts using bots, scripts and other automated methods.

The company said that since the technology was introduced last December, it has seen a 20 percent decrease in online attempts to set up new email accounts.