From: root@localhost
MIME-Version: 1.0
Content-Transfer-Encoding: 7bit
Content-Type: multipart/mixed; boundary="outer"

--outer
From: sub@localhost
MIME-Version: 1.0
Content-Transfer-Encoding: 7bit
Content-Type: multipart/mixed; boundary="inner"

--inner
MIME-Version: 1.0

a
--inner
MIME-Version: 1.0

b
--inner--

--outer
MIME-Version: 1.0

sig
--outer--