Spammers are developing new techniques to stop them being blocked by filtering software, according to email security software maker SurfControl.

The company has identified new techniques that spammers use to prey on the naivety of email users and pose significant legal, security, network and productivity risks for businesses.

Steve Anthony, sales director at SurfControl, said: "Spammers have realised that email filtering vendors are finding out the tricks of their trade and developing products that stop them."

According to Anthony, one trick used by spammers, 'counterfeit characters', involves replacing standard characters with numbers or accented characters to fool filtering dictionaries.

'V1agra' and 'M0RTG4GE' are examples of the use of counterfeit characters.

Other methods to avoid scanning see spammers breaking down URL structures, using white text on white background on HTML to disguise text, redirecting recipients to unexpected websites, putting random words or characters in subject fields and disguising format to avoid dictionary or statistical filtering.

SurfControl used covert techniques to gather this information by setting up unprotected servers, and then watching as spammers took them over to send out unsolicited email.

"We have been able to monitor what techniques are being used," said Anthony.

He added that the problems caused by unsolicited mail offered a huge opportunity to resellers.

"Businesses must keep up-to-date with the products available to stop spam, and we have training to keep our channel up-to-date so they can support their customers."

Chris Jenkins, product marketing manager at reseller Cisilion, said spam alone is not a huge area for the company.

"It is not a massive opportunity for us but we sell spam as part of a complete solution," he said.