Content-Type: multipart/mixed; boundary="1"

--1
Content-Type: multipart/mixed; boundary="2"

--2
Content-Type: message/rfc822

Content-Type: text/plain

1
--2
Content-Type: message/rfc822

Content-Type: text/plain

22
--1
Content-Type: message/rfc822

Content-Type: text/plain

333