Virus Scanning API 1.0 was introduced in Microsoft Exchange Server 5.5 Service Pack 3 (SP3) and was standard until the release of Exchange 2000. Many improvements have been made to Virus Scanning API 1.0 to improve performance with Exchange Server. When you use a VAPI scanner and a client tries to open a message, a comparison is made to make sure that the message body and attachment have been scanned by the current virus signature file. If the current vendor or signature file has not scanned the content, the corresponding message component is submitted to the antivirus software vendor for scanning before that message component is released to the client. [Source: Microsoft help & Support]
See also API.