• 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

• VS API 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 VS API (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-VS API-based solution.
• For Exchange customers using a non-VS API-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.
  
• VS API 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 normally is not noticeable, but in very high load conditions it is possible that some applications exhibits some timeout failures.

VSAPI scanners operate on individual parts that is 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.