Spammers are becoming more sophisticated in the way they present their messages to avoid spam filters, and how they harvest email addresses, according to security specialists.

McAfee's study of spam in August showed that the links in emails used 72 per cent more website addresses per hour in an effort to stop domains being identified by spam filters and weeded out. 

"It's a cat and mouse game where spammers try to change their URLs faster than the anti-spam companies can react," said Guy Roberts, development manager at McAfee.

"If it takes traditional blacklists 15 to 20 minutes to block a site, then that's how fast the spammers need to change their URLs."

Roberts explained that, because it only costs $6 (£3.20) per domain registration, spammers could spend less than $100 for four hours of 'advertising'.

Meanwhile, Sophos has warned of a new trick spammers are using to harvest email addresses for their operations. 

The security company has witnessed a spammed email campaign from a fake research firm called 'Gemma' that poses as a study into chain letters.

"Under the pretence of 'research' spammers are trying to fool internet users into passing on dozens of email addresses with every message they forward," said Graham Cluley, senior technology consultant at Sophos.

"At best this could result in spam being sent to all of your friends and colleagues, but at worst they could be put at risk of identity theft. Computer users should break the chain and not respond to messages such as this one."

The message asks users to forward on chain letters, "even the rude ones".

"This project is based over the next year and I need at least 500,000 forwards for this project to be a success, so please keep them coming, the more the better," it continues.