Last year, spammers embeded web bugs and spam beacons - pieces of HTML code - into almost half of their messages to check recipient email addresses were valid for future spamming, according to research from anti-spam firm MX Logic.
"Millions of users are unaware that spammers have the ability to track when they view and open their email," said Scott Chasin, chief technology officer, MX Logic. "This reinforces the fact that spammers are using increasingly deceptive tools to invade end users' privacy and harvest valid email addresses."
Spam beacons are a variant of web bugs, which have traditionally been used by marketing companies to measure page views and track surfing behaviour. When a user opens an email containing a spam beacon, it sends a signal back to the spammer, validating the address.
Chasin added that databases which collect the beacon data are often hosted on "zombie" machines, which makes tracking spammers difficult.
The firm examined millions of spam messages to detect the bugs and beacons. It is now attempting to block the messages for its customers.
Over the past year, the firm found that spam had increased 67 per cent.