At work we have had this issue a few time so I though it would be usefully to record the fix.
The problem occurs when a user tries to print a message from the preview pane in Outlook.
Only the default printer shows in the printer drop down list and it is disabled so the user cannot change printers.
To get around this issue the user simply has to double click on the message to open it in a new window.
Now when the user goes to file -> print it will still show the default printer in the drop down list but it will be enabled so that another printer can be selected.
Hope this helps someone.