Microsoft is to start charging for a Hotmail feature that allows Hotmail users to access their email using the Outlook email client.

Microsoft is making the move not to increase the number of paying Hotmail users but because the feature is being abused by senders of spam, says Brooke Richardson, lead product manager for MSN at Microsoft.

"Essentially what spammers do is create scripts so they can rapid-fire email from Outlook or Outlook Express and pop off a hundred emails from each of those Hotmail accounts in rapid succession," Richardson says.

"On certain days we have seen tens of thousands of Hotmail accounts set up and spamming in this matter."

To prevent abuse of the feature, Microsoft will stop making it available to new users of free Hotmail and MSN mail accounts starting this week. Current users can continue to use the feature but will be asked to become Hotmail subscribers over the coming months.

By April next year, the feature will no longer be available for free, Richardson says.

The Hotmail and MSN mail feature is known as WebDAV, after the Web-based Distributed Authoring and Versioning (WebDAV) protocol that enables it. It is enabled on about 5 percent, or 9.4m, of the 187m active Hotmail accounts, according to Richardson.

While the decision to make the link with Outlook or Outlook Express a paid service won't be welcome news to some users, Microsoft had to take the step "for greater good" of the Hotmail and email community, Richardson says.

Furthermore, rival web-based email providers such as Yahoo already charge for similar functionality, she says.

Other actions that Microsoft has taken to prevent abuse of Hotmail by senders of spam include a limit on outgoing messages of 100 per day on free accounts and an extra validation requirement when signing up for an account, Richardson says.

"We do a lot of stuff in terms of understanding the characteristics of spammers so we can watch for them and shut them down when we see them," she says.