Modifying the mail design, and the Inbox in particular in Notes is fraught with peril, even for the experts. The Inbox is a special folder (not a view), and you would have to go to fairly extraordinary lengths to accomplish what you want and not have it either fail to receive mail or revert back to the Notes design when the design process runs.
You would be so much better off, and your users would be worlds happier if you upgraded to version 7 or 8 (if your licensing situation permits it).
Mail is such a fundamental application in the infrastructure, and it’s the users’ main interface with the world. If they are seeing a version of mail that’s ten years old (R5 came out in 1999), then it will just be the object of ridicule, in spite of any tweaking you might do.
If your situation doesn’t permit upgrading, then you can search the Web for tips, and be prepared for a lot of study. The Lotus developer forums that can be found at notes.net are an excellent resource.