AOL has announced that it will start charging to send emails in an attempt to cut down on spam.

The company will now define two forms of email: AOL 'certified' and AOL 'normal'. Companies that pay for certified mail will be able to bypass spam filters, at a cost of $3 to $4 per 1,000 emails.

AOL will still send 'normal' emails but these could fall prey to spam filters if they contain URLs or attachments.

"We are creating this new class of email so that recipients can be assured that they're receiving secure emails from legitimate senders," said Richard Gingras, chairman of Goodmail Systems, which is supplying the charging technology to AOL.

Gingras explained that Yahoo will also be installing the technology to charge for emails. Between them AOL and Yahoo carry 50 per cent of private emails in the US.

"We believe this new layer of protection will widen the gap between the amount of good email we want our users to get and the dwindling amount of bad email they might get," said Barry Appelman, senior vice president and chief web strategist at AOL.

"Spam on the AOL service has now declined by over 85 per cent since its peak in 2003, but more work needs to be done."