usage: modmail -A 'Header-Line'     # add Header-Line
       modmail -a 'Header-Line'     # add Header-Line only if new
       modmail -e 'Perl-Expression' # edit header lines ($_) with Perl-Expression
       modmail -E 'Perl-Expression' # edit body ($_) with Perl-Expression
example: modmail -e '$s=$1 if /^Subject: (.*)/' -E 's/^/>>> $s <<<\n/'