Edit 4/20/2010: Replaced the post body with the link explaining how to get most current information on the subject:

Learn about Exchange 2010 Virus Scanning (VS) API 2.6.1 and VSAPI TAP

The VSAPI 2.6.1 provides programmatic interfaces for a Virus Scanning application to access items and attachments stored in Exchange. The VSAPI 2.6.1  is only provided to organizations enrolled in the VSAPI TAP program.

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Hi, <br>Maybe this questions belongs somewhere else and hopefully you can point me in the right direction... <br>How does one acquire a copy of the VSAPI? We are a certified Microsoft Solution Provider but I have not been able to find it anywhere on the partner site or the MSDN site. Any help would be appreciated. <br>You can email me the response at vijayk@inetsupport.com <br> <br>thanks. <br>
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Does AV software using VSAPI increase the RPC Average latency for large MB servers?
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The answer is that it shouldn't increase the RPC average latency if the scanning engine and the appropriate configuration of scanning threads are made. However, if there is an incorrect number of threads, the scanning engine is performing poorly, or if there are disk performance issues, then when a client request a message that has not been scanned, they will experience a delay waiting for the scan to complete.

I would ExBPA to validate the configuration of VSAPI, then look into disk latency issues. Of course the tell-tell sign of clients waiting is a sustained number of items in the scanning queue.
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we have a problem , when background scanning is enabled a large number of items hit into the AV engine . It is appx 4000 items /min when background scanning is enabled and it is 400/min when it is disabled. <br>Any idea why it would be occuring ?