The HEADER, HEADER.FIELDS, HEADER.FIELDS.NOT, and TEXT part
specifiers can be the sole part specifier or can be prefixed by
one or more numeric part specifiers, provided that the numeric
part specifier refers to a part of type MESSAGE/RFC822.
So, x.HEADER can be used only with message/rfc822 parts. The example
from RFC 3501:
4.2 MESSAGE/RFC822
4.2.HEADER ([RFC-2822] header of the message)
4.2.TEXT ([RFC-2822] text body of the message) MULTIPART/MIXED
Fetching 4.2.MIME could return something like:
MIME-Version: 1.0
Content-Type: message/rfc822
While 4.2.HEADER would fetch the actual message's headers.
-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 196 bytes
Desc: This is a digitally signed message part
URL: <http://mailman13.u.washington.edu/pipermail/imap-protocol/attachments/20070311/fd783759/attachment.sig>