Make Outlook email messages more searchable

So I was looking into combating the problem of clients replying to the wrong emails about projects. Often time I get a client who just replies to the last email I sent them even though that subject has nothing to do with what they are talking about (drives me crazy). I found this tip on +Lifehacker just now and it totally blew me away. Apparently in Outlook you can change the subject of emails you received. This fixes my problem and it allows me to start adding tags into my emails for future searches.