MBOX-Line: From RFC 3501:
To: imap-protocol@u.washington.edu

         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.

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