Often, when working with mailing-lists, I want to step into thread (which I am
not able to hit reply/reply-to-all because I don't have that particular email
in mailbox) and do not want break it -- so I want explicitly specify the
message-id I'm replying to.  Thanks for considering, that would be so perfect
enhancement.

As an example, look at `git send-email --in-reply-to=""`, I'm not aware of any
mail client which is able to do this.  Examples are welcome.

Pavel