Microsoft's Thread-Index Field.

[Ralph Corderoy]

Hi,

In wondering why Microsoft Outlook users seem to find handling email
conversations so difficult, I recently found out about Microsoft's
‘Thread-Index’ field.  Outlook uses it for threading instead of
Message-ID, References, or Subject.

I'm guessing that by replying to an Outlook email and not sending back
the same Thread-Index field, my email appears as a new thread?
Who knows, but it does seem non-MS MUAs and email-generators like
Bugzilla have jumped through hoops so those poor Outlook user don't have
to suffer yet more.

To see if you've any emails with the field:

    pick --thread-index 'z*'

To return any header in a reply, repl(1)'s -form file can have something
like:

    %<{thread-index}Thread-Index: %{thread-index}\n%>\

Should we modify nmh's standard repl format files to have this?

Which leaves supporting generating a new Thread-Index for comp(1), and
whether that's required.  Or does Outlook add one to every email which
arrives without one?

Obviously, the user could add the header in their editor, leaving the
responsibility of how to generate it up to them.  Or mh-format(5) could
grow features, e.g. base64.

I'm short on time at the moment, so I'll just dump some links which I've
found from research so far.

- What does Outlook need to recognize a thread?
  
https://superuser.com/questions/1351430/what-does-outlook-need-to-recognize-a-thread
- The bug on Bugzilla mentioned in the previous link.
  https://bugzilla.mozilla.org/show_bug.cgi?id=411601
- Some Microsoft documentation on the content of the field.
  
https://docs.microsoft.com/en-us/openspecs/exchange_server_protocols/ms-oxomsg/9e994fbb-b839-495f-84e3-2c8c02c7dd9b
- And the PtypTime data type it refers to.
  
https://docs.microsoft.com/en-us/openspecs/exchange_server_protocols/ms-oxcdata/0c77892e-288e-435a-9c49-be1c20c7afdb
- A Python implementation for Outlook's thread-index headers for message
  threading; not checked by me for correctness.
  https://gist.github.com/brettp/f440bbe4e5c383cefda3

-- 
Cheers, Ralph.