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: