#!/bin/sh
t=/tmp/$$.eml
cat > $t
# e.g., From: Christopher Faylor via Test-list <test-list@sourceware.org>
from=$(sed -nre 's/^From: (.*) via .* <.*$/\1/p' $t)
# e.g., Cc: Christopher Faylor <me.test-list@cgf.cx>
cc=$(sed -nre "s/^Cc: ($from "'<.*>).*$/\1/ip' $t)
exec < $t
rm $t
exec sed -e "s/^From:.*/From: $cc/" -e "/[Cc][cC]: $cc.*/d"