I am creating Transport Rules to create company-wide signatures. One of the quirks that I noticed was that, while everything looks great for HTML mail, anything sent in plain-text includes a bit of ugly HTML code (eg. [a href] tags, etc).
I tried getting around that with a condition that Message Header "Content-Type" includes "multipart".
Unfortunately, that doesn't work. I believe it fails because all HTML emails include two Content-Type headers. The first for HTML, and the second for fall-back to plain-text. Exchange seems to only count the final "Content-Type" header... which is always "plain-text".
So.. does anyone know how to apply a Transport Rule only to HTML emails, without requiring a third-party product?