Content-Type: multipart/mixed; boundary="AAA"
From: Mail Delivery Subsystem <xxx@example.com>
To: yyy@example.com
This is a MIME-encapsulated message
--AAA
Stuff
--AAA
Content-Type: message/rfc822
From: webmaster@python.org
To: zzz@example.com
Content-Type: multipart/mixed; boundary="BBB"
--BBB--
--AAA--