* Text only mails are created like this:

cat mail/header-$lang > tmpfile
cat mail/mail-$lang.txt >> tmpfile

* text+html+images mails are created like this:

cat mail/header-$lang > tmpfile
cat < EOF >> tmpfile
Content-Type: multipart/related; type="multipart/alternative";
       boundary="=-tThpx1YEZqL4gn53WjQ1"

--=-tThpx1YEZqL4gn53WjQ1
Content-Type: multipart/alternative; boundary="=-aFPGjTr5jUHhXPWxbLcT"

--=-aFPGjTr5jUHhXPWxbLcT
EOF
cat mail/mail-$lang.txt >> tmpfile
cat mail/mail-$lang.html >> tmpfile
cat mail/mail-images >> tmpfile