Recommend adding the following paragraph (excuse the XML markup):
<t> There are tradeoffs in the decision of what constitutes
the "core" of the message, which for some fields is a
subjective concept. Including fields such as "Message-ID"
for example is useful if one considers a mechanism for
being able to distinguish separate instances of the same
message to be core content. Similarly, "In-Reply-To"
and "References" might be desirable to include if one
considers message threading to be a core part of the
message. </t>
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