You can direct Emacs to insert certain default headers into the mail buffer by setting the variable @code{mail-default-headers} to a string.  Then @kbd{C-x m} inserts this string into the message headers.  For example, here is how to add a <del>@samp{Reply-to}</del> <ins>@samp{Reply-To}</ins> and @samp{FCC} header to each message: