I recently came across a problem after migrating a company from Exchange 2003 to Exchange 2010… none of the Outlook 2003 clients would connect to the server once their mailbox had been migrated. Other clients (MS Outlook 2007 and 2010) would work perfectly… so I knew this wasn’t necessarily a server-based problem…
After some digging around I came across this:
This screenshot is from the advanced settings section of an Exchange connector in Outlook 2003. All that’s needed to be changed here, is to put a tick in the “Encrypt data between Microsoft Office Outlook and Microsoft Exchange Server”. Once you’ve clicked OK, and saved the changes to the profile, Outlook works properly again.