The simplest ideas are sometimes the most effective. MessageLabs, a division of Star Technology Group, may be a case in point.

Recognising that maintaining anti-virus software on every PC that connects to an organisation's network is an awkward, time-consuming business, Star launched an outsourced scanning service in 1999 that examines incoming emails for viruses before they hit the corporate mail server.

The service was an instant hit. By January 2000, Star had set up MessageLabs as a separate business unit that focuses primarily on the anti-virus service, but also offers an increasing range of content filtering services, such as scanning for spam and pornographic images.

Today, MessageLabs has one million end-users on four continents, no doubt attracted by MessageLabs' claim that it has not let one virus slip through its net in almost three years of operation.

It has achieved this feat, says co-founder and marketing director Jos White, by deploying not one, but four anti-virus scanners. Off the shelf, it uses anti-virus software products from McAfee, F-Secure and Cybersoft. This is augmented by Skeptic, an anti-virus engine the company developed in-house.

Skeptic's strength, says White, is its heuristics engine. This is a technology for examining incoming email attachments and assessing their ability to do harm to an end-user's PC, based on a collection of rules, such as whether it contains Visual Basic Script or needs to access the user's email address book – both common signs of nefarious intent.

Using heuristics technology, Message-Labs can block viruses before traditional anti-virus software vendors have identified new viruses and issued a ‘signature' to enable their software to identify them.

However, it will need to move fast to consolidate its position. Anti-virus software vendors have not only started to add heuristic capabilities to their software, but are also starting to offer managed email filtering services themselves.