"Blocking is only suited to a trusted environment where there is an existing relationship between sender and receiver, [while] the benefits of e-mail is anyone can e-mail anyone - this is the beauty and the weakness of the system," Warrilow said.

"We really haven't seen that much success with that type of blocking approach - the only exception is where the mailing relays and ISPs use it as the back door."

He said the most popular approach is blended filtering solutions as they do not restrict legitimate e-mails.

"An heuristic approach using different filters and algorithms is needed to fill the cocktail base - that's the approach most people have taken because they need to ensure people without a pre-existing relationship can e-mail them."

Sophos head of technology for Asia Pacific Paul Ducklin agreed, saying a mixed approach is needed, because a complete, single antispam method does not exist.