I've posted several posts regarding various issues we are finding in the process of migrating from Exchange 2003 to 2010. All clients are currently Outlook 2003 - upgrading this is not an option at the moment. The latest issue is when some users still on an Exchange 2003 mailbox server try to open a resource mailbox on the 2010 server intermittently get the error: "The connection to the Microsoft Exchange Server is unavailable. Outlook must be online or connected to complete" We had a similar problem with users on a different server and applied hotfix: http://support.microsoft.com/kb/979690 This issue was constant, though, in that users could not open more than one other mail folder. This new issue is more intermittent and users can have several mail folders (calendars etc) open before getting this problem. I've found reference to the RCAMaxConcurrency throttling value and increased this but this has made no difference. http://social.technet.microsoft.com/Forums/en-US/exchangesvrclients/thread/6d79d0b7-0aa7-4689-925c-c0eb37c59780 http://ctrlshiftescape.net/?p=81 Advice welcome Thanks

Outlook 2003 connects to Exchange by default using unencrypted MAPI. There is an option to enable encryption in the account settings, but it's not enabled by default. When Outlook reaches across to the 2010 environment, it may fail because Exchange 2010 requires MAPI encryption by default. Can you check this setting in Outlook 2003? Mike Crowley Check out My Blog!

Hi, Is the issue occurred on Outlook 2007, Outlook 2010? And also please verify that the hot fix is actually installed by checking the version of Dsproxy.dll. The hotfix should have updated the version to 6.5.7656.1 Thanks Allen

Hi Thanks for your replies. I forgot to mention that the encryption setting has been set on the mail profiles so this is not the issue. Also, as I mentioned, all clients are on Outlook 2003 and we cannot currently upgrade these. Since I posted the original question I have installed the hotfix on the server where the users are experiencing the problem. It was already installed on a different server. I am waiting to hear back from the users to see if the problem is still occurring. Thanks

If you still experience issues after following up these patches and throttling policy changes you may need to wait until Exchange 2010 SP1 is released. Steve Steve Goodman Check out my Blog for more Exchange info or find me on Twitter