Hotmail, the free web-based email service run by Microsoft, has introduced measures to crack down on spam.

Over the past two weeks the company has been implanting a rule limiting to 100 the number of email addresses a sender can target in any 24-hour period.

Spammers typically use free email services such as Hotmail to mass mail their advertisements to unwary web users. The ads promise anything from Viagra to pornography by way of get-rich-quick schemes.

Experts predict that up to 50 per cent of emails travelling over the web at any one time are spam.

To curb the abuse, users of Hotmail's free service are limited to sending emails to 100 addresses per day.

According to business news website Forbes.com, Microsoft said that the limit is well within the level that an average person would be able to send via email. Users of the Hotmail paid-for service will have no such limitations.