What if the recipient wants to refer back to that seventh JPEG of nine?
They would probably say, "Oh, it's the one with the white cat making a
funny face". I mean, I understand WHY that header exists, I just don't
think that it is ever used that way ... WITH the exception of text/html
parts referring to included images for rendering. I'm trying to think of
people who would a) refer to a particular part using a Content-ID header
and b) are NOT on this mailing list.
I had a shufty at emails here. Ignoring those from nmh, I found
Content-ID in the email's headers of other MUAs. Pine was one, and
Microsoft do it too, on occasion.
I was curious about pine, so I looked at that. It does generate them,
but you can turn that off. The comments are:
/*
* If requested, strip Content-ID headers that don't look like they
* are needed. Microsoft's Outlook XP has a bug that causes it to
* not show that there is an attachment when there is a Content-ID
* header present on that attachment.
*
The same applies to Alpine. It looks like those are only generated for
"attachments", rather than every MIME part.
BTW, Paul Fox, hi Paul, sent some, with nmh I presume, where one "host"
was shorter than the other, e.g.
Content-ID: <8023.1476240963.1@grass>
Message-Id:
<20161012025603(_dot_)0D99E51810FF(_at_)grass(_dot_)foxharp(_dot_)boston(_dot_)ma(_dot_)us>
I suspect the one was created by nmh, the other created by Paul's first-hop
MTA (-nomsgid is the default for send).
AFAICT, Content-ID is only required for a message/external-body type (hm,
we MAY not get that right).
--Ken
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