I had a user who upgraded from a previous version of Outlook to Outlook 2010. Whenever they open an Outlook email they recieve an error "Initializing MAPI session object failed." After clicking OK the email opens.
This is resolved by going to C:\Windows\System32 and renaming mapi32.dll to mapi32.dll.old. Then run a tool called fixmapi.exe under C:\Windows\System32. This will generate a new mapi32.dll file.
Note: Sometimes Administrators do not have rights to rename or modify mapi32.dll, only "TrustedInstaller" does. In cases like this you need to take ownership of mapi32.dll and provide Administrators access to the file.