Troubleshooting Exchange 2007 9646 Errors

Friday, February 22, 2008
I client has users who have been migrated from Exchange 2003 to Exchange 2007 SP1, running on Windows Server 2003 SP2.

After a while, users are no longer able to connect via Outlook to Exchange - OWA continues to function, but Outlook (2K3 and 2K7) stops working.

This is because of a new feature in Windows 2003 SP2 that enables "Scalable Networking" - In short, it shuts down closed connections to the server, but it doesn't play well with Exchange. When Outlook connects over several MAPI sessions, the unused ones are shut down by Windows, but they aren't closed cleanly and Exchange still sees them as open sessions.

Once the user has 32 open sessions (a combination of valid and invalid ones) - Exchange cuts them off and event ID 9646 errors are seen on the mailbox server event log:
Mapi session "/O=BLATHER/OU=PACIFICA/cn=Recipients/cn=CooperH" exceeded the maximum of 32 objects of type "session".

A hotfix will be released in late March that addresses the issue, but the short term fix is to run the following command from the command line on all Exchange 2007 mailbox servers:

Netsh int ip set chimney DISABLED

The following articles discuss the technology and the issue:


  1. Hey Jeff. Funny, I googled this and ended up on your blog! I tried the netsh command in Exchange 2007 on Win2008 and it doesn't exist (the "set chimney" isn't an option for netsh. There is precious little out there for this error on windows 2008.
    Cheers, Amanda

  2. Hi Amanda,

    The chimney features are disabled by default in Windows 2008.


Thank you for your comment! It is my hope that you find the information here useful. Let others know if this post helped you out, or if you have a comment or further information.