Outlook 2003 error after Exchange 2010 transition

If you get this error when you try and open legacy Outlook clients after a transition to Exchange 2010 you need to open the Outlook profile and click 'More options' and 'Security' and enable "Encrypt data between Microsoft Office and Microsoft Exchange Server"

This does not effect Outlook 2007 and 2010 clients as they connect another way.

See http://support.microsoft.com/kb/2006508 for more info, medthod 3 might be worth rolling out of you have a lot of legacy Outlook clients