Spammers are turning to tactics favoured by virus writers to get their unwanted messages into circulation.
A tidal wave of spam could be flooding from your in-box
Anti-spam activists have found that some unscrupulous spammers are hijacking the e-mail accounts of innocent users to send millions of messages.
The spammers take over the accounts using malicious e-mail messages that resemble computer viruses.
As efforts to beat spam accelerate, many junk marketers are keen to cover their tracks and hide the real origin of the messages they want to send.
As governments, companies and net service firms unite to tackle spam, the number of insecure internet relays and mail gateways available for spammers to use is diminishing.
This has driven some desperate junk mailers to start creating virus-like programs that take over someone's e-mail account and use it as a funnel for millions of messages.
Like many other viruses these programs exploit weaknesses in Microsoft's popular Outlook e-mail package.
One of the first hijacking programs to emerge was called "Jeem" and hid within it an e-mail engine so it could efficiently route messages via an infected computer.
Another, dubbed Proxy-Guzu, arrives as a spam message with another file attached.
Clicking on the attachment makes it contact a Hotmail account with information about the infected machine making it possible for someone to route mail through that computer.
Any attempts to trace the original source of the flood of spam sent out by this method will only return the net address of the innocent computer used to send it.
"Spammers are beginning to use virus-like techniques to cover themselves," said Larry Bridwell, content security programs manager at researchers ICSA Labs.
A Monty Python sketch led to unwanted mail being called spam
"Spam is one of the two things that the security industry is going to be asked to deal with," he said, "The other is adware or spyware."
Many businesses have their e-mail vetted before it is delivered to employees to stop viruses striking.
Some companies, such as Blackspider Technologies, are starting to offer e-mail filtering services that block both viruses and unwanted commercial messages.
Net-based technology magazine Security Focus was the first to write about the spam trojans.
Programs like this are called trojans because, like the wooden horse of antiquity, they conceal a pernicious payload inside an innocent looking wrapper.
Similar techniques have been used by malicious hackers who want to carry out denial-of-service attacks.
Staging such an assault involves the recruitment of hundreds of innocent machines using trojans and then telling them to bombard a particular website with bogus data packets.