E-mail is one of the primary carriers of malware, and signature-based filters alone have troubles catching up with their fast, outbreak-like spread.
In order to block malware before they hit mail servers and clients, Halon Security's mail gateway uses three anti-virus filters; an outbreak-based filter from Commtouch which doesn't really scan for viruses, but rather blocks messages which is sent in large volumes and contains suspicious content such as executable code, the award-winning signature-based anti-virus from Kaspersky and the open source ClamAV. Together they protect you during all phases of a virus' life: from when they are only beginning to spread until they fade away.
Simply put, it prohibits e-mail with executable code that is distributed in large quantities, thus detecting new viruses for which signature-based engines has not yet definitions, in addition to traditional anti-virus filters. Rest assured that no viruses will get passed through.