A debian user reported (http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=575423) that mail header parsing in the compose mail helper was case sensitive, so CC is for example not recognized. He provided a patch (against exo 0.3) which is available at http://bugs.debian.org/cgi-bin/bugreport.cgi?msg=5;filename=exo-compose-mail-0.3.diffs;att=1;bug=575423
Applied in 21503ab.