Look. I don't know if this is the right place to "advertise, but I'm going to risk it.
I have been trying an add-on for Mail on the Mac called "Mail-Tags". It allows you to add all kinds of keywords, which can then be used in Spotlight and Smart Folders. My main use has been to keep track of serial numbers, receipts, things to pay and so on. I have just donated the suggested "shareware" fee as I know I will get greater use when I implement GTD properly ("COME ON AMAZON! The library wants their book back!")
If you want to try it ... or use without donating ... try it at http://www.indev.ca/MailTags.html
I have no affiliation with the developer ... I just know a good thing when I see it.
If this is completely wrong of me, I aplogise, but I think it is worth sharing anyway!