I think the guy behind MailMate¹ has done just that. It’s definitely the most powerful mail client on OS X. I ended up buying it, at least!<p>――――――<p>¹ — <a href="http:&#x2F;&#x2F;freron.com&#x2F;" rel="nofollow">http:&#x2F;&#x2F;freron.com&#x2F;</a>