I have a user utilizing Outlook 2011 on her MacBook OS 10.6.8 who is unable to edit the subject line of received/sent emails. Actually the option to 'Edit Message' under Message in the Outlook tool bar is not even available.
She is syncing these messages with MS Exchange. Not sure if this has anything to do with it. I could see if the changes were not saved but she cannot even overwrite the subject at all.