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--