From: a@example.com
Subject: blargh
Message-ID: <multipart@example.com>
In-Reply-To: <irp@example.com>
MIME-Version: 1.0
Content-Transfer-Encoding: 7bit
Content-Type: multipart/mixed; boundary="b"

--b
Content-Type: text/plain

hi
--b
Content-Type: text/plain

bye
--b--