This update has been around since February but I’ve only come across this recently.

Symptoms

Users get “The set of folders cannot be opened” error in outlook when trying to access an additional mailbox automapped from Exchange 2010. The same users can access the mailbox if it is added manually (through Account Settings). Other users may be unaffected all together.

Cause

It appears that following the rollup update 5 automapped mailboxes can only be accessed using Outlook SP2 and higher. If you check your office version number you’ll likely find you’re either on RTM or SP1.

Office 2010 RTM 14.0.4763.1000
Office 2010 SP1 14.0.6029.1000
Office 2010 SP2 14.0.7015.1000

Resolution

Update MS Office to SP2.