The most common reason to send a message while in Rmail is to reply to the message you are reading.  To do this, type @kbd{r} (@code{rmail-reply}).  This displays a mail composition buffer in another window, much like @kbd{C-x 4 m}, but preinitializes the @samp{Subject}, @samp{To}, @samp{CC}, <del>@samp{In-reply-to}</del> <ins>@samp{In-Reply-To}</ins> and @samp{References} header fields based on the message you are replying to.  The @samp{To} field starts out as the address of the person who sent the message you received, and the @samp{CC} field starts out with all the other recipients of that message.