Some email is 'bugged' to alert senders when messages are opened, researchers say.

Hidden code in email messages is increasingly being used to track the success of spam campaigns, according to a warning by an antispam technology company.

Up to 50 percent of all spam released in the last year is bugged with so-called spam beacons that send a coded message back to the spammer whenever a spam message is opened, according to US-based MX Logic.

Such tracking helps spammers refine their distribution lists and weed out bad email addresses from good ones.

The beacons, also known as web bugs, are created with HTML code embedded in the email.

For example, the beacon may be a URL for an image file that is stored on a server controlled by the spammer.

When the email message is opened, the email application requests the image and sends along an encoded email address of the recipient.

The spammer's server responds by sending the image file to be displayed, but it also captures the email address that was sent in a database of "good" addresses, according Richard Smith, an independent computer security consultant.

MX Logic analysed millions of spam messages that it processes for its 1500 customers each day to study the spam beacon problem, according to Scott Chasin, MX Logic's chief technology officer.

"MX Logic's products use heuristic analysis to spot and block messages containing spam beacons," he says.

The company says renewed awareness of the spam beacon problem is needed because most email users don't realise that they are being tracked by spammers. Also, many email providers are not interested in stopping a 'feedback loop' that lets spammers improve their art.

MX Logic found that spammers are becoming more sophisticated in hiding the spam beacons from antispam filters. Also, the spammers use the data reported by the beacons to groom their messages and evade detection, according to Chasin.

The databases that collect the beacon data are often hosted on compromised "zombie" machines, making it difficult to track the spammer responsible for a particular campaign.