Virus Scanning Software
Email attack vectors can usually be stopped by virus-scanning software. These usually operate by looking at the attachments in incoming email, looking for the signatures of known viruses.The most important aspect of this is that the virus definitions be kept up-to-date. In large organizations, virus protection for non-mobile computing resources (i.e. desktop computers that don't roam to external networks the way laptops do) can be concentrated on the server using server-side virus-scanning tools. While this reduces the management overhead for keeping virus definition up-to-date, it has the added benefit of stopping outbound virus mail if someone should happen to get infected.
There is another emerging method of protecting against email virus/worm propagation: propagation profile filtering. By using certain features and non-standard behavior in the way that viruses and worms propagate themselves, it's possible to profile and filter them before the mail is even accepted. This is very similar to the transport level filtering done for spam as described above.
The virus-scanning market is a crowded, somewhat mature space. However, one interesting new player that has emerged is ClamAV an Open Source solution to server-side virus scanning. This upstart project is now ranked fifth in terms of response time to virus outbreaks; this is well ahead of the large players like MacAfee, Trend Micro, and Symantec.
Network Firewalls
The network firewall deployed on individual machines, as opposed to the single point of exit/entry firewall for an entire network is a newcomer to the security scene. These software products are becoming somewhat ubiquitous, with Microsoft even adding the native capability as an integral part of Windows XP. While being a valuable and increasingly necessary piece of the security of puzzle, they have a small role in email security and quality enhancement.