Spam messages are on the rise, despite the threat of legislation to curb this mailbox menace.

Antispam firm Brightmail's probe network, which accesses around 100 million mailboxes globally, recorded 4,971,097 spam messages during July. And it expects the figure for August to exceed five million.

Brightmail is sceptical about the effectiveness of new European legislation, due to come into effect by October next year, that will allow users to opt-in and request such junk mail.

"If you viewed spam as a European problem then the legislation should have a huge effect. Unfortunately it is not," said Gert Veendal of Brightmail Europe.

"Spam needs to be viewed on a global scale as most of it comes from outside Europe. Therefore global rules would need to be developed before any real change occurs."

But Brightmail recognises the internet is difficult to police. There is currently no legal European definition of a 'spammer' or the punishment they would face if caught.

"Most spam cannot be traced. Therefore, if a spammer's emails are blocked he can simply use another address. Until we have the technical means to trace the origin of the spammer it's difficult to control it," said Veendal.

The other problem is that spam mail comes in many forms. According to Brightmail only about a quarter is marketing, yet this is the sector likely to be most affected by the legislation.

The majority of spam is sexually explicit mail — usually in the form of messages directing the recipient to pornographic sites.

"People are becoming annoyed with spam, but it is not so much the amount they receive as the content. Although people have become used to receiving spam it has definitely reached nuisance levels," Veendal added.

Jupiter Research predicts 10 percent of all messages reaching UK inboxes are spam.