Googling Your Email
Subject:   Notes client does this somewhat
Date:   2002-10-08 05:54:15
From:   anonymous2
"For years, I've hoped email clients would begin to support custom metadata tags that would enable more robust contextualization"

Notes/Domino gives access to smtp X-fields so you can for example categorize on one, but i dont think anyone know :)

Disclaimer: Not sure if it does today, this was in version 4 (I think was a while ago)