The global junk mail plague is to get dramatically worse as criminal spammers take control of victims' PCs and use them as anonymous proxies to send email via their ISPs' mail relay.

According to anti-spam organisation Spamhaus the recent increase in this proxy-spam activity is caused by newly engineered versions of stealth proxy-spam software released by spammers.

"New versions of proxy packages released by Russian spammers operating in the US now have a feature which instructs the hijacked proxy to send the spam out via the mail relay of the ISP to which the proxy is downstream," Spamhaus warned.

Before this explosion in proxy-generated spam, most email traffic arriving at ISPs' mail servers came mainly from two sources: sent directly by the spammer, or sent by the spammer through a hijacked computer (proxy).

These two sources have been relatively easy to deal with, as they can both be blocked. But Spamhaus warned that the source of incoming spam is changing, and ISPs are seeing far more spam coming directly from the major mail relays of other ISPs.

AOL, one of the first to notice the change, now reports that over 90 per cent of its incoming spam comes directly from other ISPs' mail relays.

"Spamhaus sees this change and the increase in spam it is producing as a threat to be taken seriously," the company stated.

"At the current pace spam could reach 95 per cent of all email traffic by mid-2006, when we would see the beginning of a slow meltdown of email delivery systems caused by overloaded queues and stressed filters."

The organisation advised ISPs to take protective measures, including throttling the outgoing mail from the ISPs of broadband customers, separating incoming and outgoing SMTP servers and mandating email authentication for all customers.