Some Outlook clients suddenly cannot resolve Exchange 2010 server

ad1) Please be more specific with sync issues. Do messages disappear? Do messages get duplicated?...

ad2) The basic issue is that Outlook 2003 support UDP and polling notifications. Exchange 2007 supports UDP, polling and Async notifications. Exchange 2010 only supports polling and Async notifications. This means when Outlook 2003 move from Exchange 2007 to Exchange 2010, Outlook clients will fall back to polling which by default only gets notifications every 30secs-1min. This means any change won’t show up immediately.

UDP notification support was removed from Exchange 2010. As a result, Outlook 2003 can only use polling notifications in online mode, which are still supported by RPC Client Access. This will result in a slight delay in updates to item status (30 seconds on average up to a 1 minute delay) when changes are made to items in a mailbox accessed by Outlook 2003.

source: http://social.technet.microsoft.com/Forums/en/exchange2010/thread/d5a610d5-b3cd-40a5-8653-634f3b174f4e

W/o the actual error message it's fairly difficult to give an accurate diagnosis, but I'm going to invoke some psychic powers and see if I can come up with something useful for you.

I've had a couple of situations where fairly old Exchange 2003 installations had Outlook 2010 introduced and the users of Outlook 2010 reported problems sending messages. The root cause was that the named property quota had been reached on the Exchange 2003 server and Outlook 2010, requiring the ability to create some named properties, was failing in a very non-graceful manner.

Review the Application Event Log on the Exchange Server computer and look for event id 9667 from the source "MSExchangeIS". This error will show the details of the named property that Outlook 2010 is attempting to create.

You should do a couple of things:

• You should install the hotfix described in this article and set the appropriate registry settings to prevent exhaustion of the named property quota with "promoted" Internet X-headers again.

• Follow the steps outlined in this article to increase the named property quota.

This will permit Outlook 2010 to create the named properties it needs to function properly and will prevent future exhaustion of the named property quota by Internet X-headers being "promoted" into named properties.

You can read some good background about this the Exchange Team blog.

(As an aside, both Blackberry Enterprise Server and Microsoft's own ActiveSync also fail in non-graceful ways when not permitted to create named properties subsequent to their installation.)