IMAP MIME part specifiers

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

MIME presenters will ignore this text.
--a
Content-Type: multipart/mixed; boundary="b"

MIME presenters will ignore this text.
--b
Content-Type: text/plain

Lorem ipsum...
--b--
MIME presenters will ignore this text.
--a
Content-Type: message/rfc822

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

MIME presenters will ignore this text.
--c
Content-Type: text/plain

Lorem ipsum...
--c--
MIME presenters will ignore this text.
--a
Content-Type: message/rfc822

Content-Type: message/rfc822

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

MIME presenters will ignore this text. Also, no IMAP-designated section begins or ends around here.
--d
Content-Type: text/plain

Lorem ipsum...
--d--
MIME presenters will ignore this text.
--a--
MIME presenters will ignore this text.