One type of anti-virus scanning available for Exchange is MAPI-based scanning. MAPI-based scanners log on to each mailbox to scan them for viruses. While this will enable you to detect and clean or remove infected messages and attachments, it is a slow, resource-intensive process.
Instead of using MAPI-based scanning I recommend that you use Exchange-aware anti-virus software that supports the Virus Scanning API (VSAPI) version 2.0 found in Exchange 2000. VSAPI, also known as the anti-virus API (AVAPI) and the virus API (VAPI), provides on-demand, proactive and background scanning of incoming messages and their attachments for malicious payloads.
This was first published in February 2004