From: user@domain.org
Date: Sat, 24 Mar 2007 23:00:00 +0200
Mime-Version: 1.0
Content-Type: multipart/mixed; boundary="foo
 bar"

Root MIME prologue

--foo bar
Content-Type: text/x-myown; charset=us-ascii

hello

--foo bar
Content-Type: message/rfc822

From: sub@domain.org
Date: Sun, 12 Aug 2012 12:34:56 +0300
Subject: submsg
Content-Type: multipart/alternative; boundary="sub1"

Sub MIME prologue
--sub1
Content-Type: text/html

<p>Hello world</p>

--sub1
Content-Type: text/plain

Hello another world

--sub1--
Sub MIME epilogue

--foo bar--
Root MIME epilogue