I created a new group (using NetInfo Manager) and added myself and my wife to it. I then changed the group ownership of my Microsoft User Data folder (and all its enclosed items) to this group. Finally, I created a symbolic link in my wife's Documents folder called "Microsoft User Data" that points to my own "Microsoft User Data" folder. This seems to work well.
Other users on our computer (ie our kids) have their own User Data folders which are completely separate from ours. They can't read our mail because they are not part of the special group.