What is and why use VS API (Virus Scanning API)?
Exchange 2000/2003/2007 provides a new virus scan API that is implemented at the lowest level in the Exchange store. This allows a virus scanning application to run with high performance and guarantees that the message will be scanned before any client can access a message or attachment.

In addition, VSAPI reduces scalability issues that can arise when a particular server has a large number of users/mailboxes. VSAPI's real-time scan allows messages and attachments to be scanned once before delivery, rather than multiple times determined by the number of mailboxes the message is delivered to. This single-instance scanning also helps prevent messages from being rescanned when a message is copied. MailScan for Exchange Server 2000/2003/2007 has the following features:

• Content Scanning.
  
• Proactive scanning.
• Priority-based queuing.
• Multithreaded queue processing.
• Per-Messaging Database configuration options.
  
• Enhanced background scanning.
  
• Virus scanning API-specific Performance Monitor counters.
  

Advantages of using VSAPI Layer
Microsoft strongly encourages the development and adoption of Exchange VS API-based anti-virus solutions

• VSAPI is secure and preserves the integrity of the Information Store and its databases.
• The Microsoft Exchange product group is committed to providing enhancements to this API, bug fixes, documentation, and technical assistance to ISVs using VSAPI (as appropriate).
• Antivirus solutions using Extensible Storage Engine API or any other undocumented API may corrupt the Information Store and its databases.
• The Microsoft Exchange product group does not provide any code updates, documentation, or technical assistance to address issues related to the use of a non-VSAPI-based solution.
  
• For Exchange customers using a non-VSAPI-based solution, Microsoft Product Support Services may ask the customer to uninstall/disable the anti-virus solution to help identify issues, this may delay in final resolution.
  
• VSAPI enhances the current core feature set by providing abilities to optimize and configure the scanning process at multiple levels, as well as providing Exchange administrators with built-in functionality to monitor the performance of the new API.
  

Limitation of using VSAPI 2.0
Since the VSAPI interface guarantees that emails cannot be accessed until scanned, on-demand scanning could cause some access delay. This delay is normally not noticeable, but in very high load conditions, it is possible that some applications exhibit some timeout failures.

VSAPI scanners operate on individual parts that are attachment and bodies.

Improvements in using VSAPI 2.5.0
VSAPI 2.5 provides more information on the email sender and recipients over VSAPI 2.0 and VSAPI 2.5 has the capability of gateway level scanning.