[tomwalsham]

Sending HTML-only email is still a large spam flag, even at major ISPs, independent of the concerns about client compatibility.

At PostageApp our solution to the problem is a templating engine where you can write your HTML/CSS separately and we inline it at run time to ensure client compatibility. We then provide a separate Plaintext tab with an 'import from html' function which does most of the work for you for managing dual formats.