Allen, great job (as usual), I'm sure I'll be using this quite often.
It would be great to add some of this to the language grammar so context-sensitive commands could be developed, and it could be pretty- ed up.
I would imagine that the existing support for the headers; to, cc, bcc, reply to, and subject; would be adequate, although I am curious if there are others that people would want to use (or could use). I also wonder if it might be beneficial to quote the values, just to make sure nothing goes haywire. But I can see that not really being necessary.
Something really interesting would be a way to lookup email addresses in the users' address book, though I have no idea if this is even possible.
--oliver
P.S. Yes, I used the command to compose this message. And it was great.