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The problem that I have is simple. We have recently upgraded from Exchange 2003 to Exchange 2010. Everything went fine, and there are very few complaints. In Exchange 2003, some of our OWA users liked the Threading view, these users love the conversations view, especially with the cross-mailbox threading.

The problem is that some of these users are now complaining that old e-mails that they got from systems like bugzilla were migrated across correctly threaded, but the new e-mails are not being correctly threaded.

If I look at the mail source using an IMAP client then I can see that all of the mails that are turning up as being threaded have the (Microsoft specific) Thread-Index header, and the mails that are not getting grouped do not have this header.

The question is, how can I make the webmail client respect the normal threading?

That is, is there a way to

  1. Make the OWA client use the standard In-Reply-To and References headers, or
  2. Make Exchange generate the Thread-Index headers for Outlook and OWA to use?
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up vote 2 down vote accepted

Sadly, there's no way to do this at present. Exchange, not OWA, generates the thread indices, so there's no way to do #1. The parameters that Exchange looks at aren't adjustable, so you can't do #2. This has bitten me many times, as threading gets broken for mailing lists generated by Google Groups, Yahoo Groups, et al.

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A possible explanation is that your Exchange 2003 installation had this hot fix installed. I have yet to discover if such a fix exists for Exchange 2010.

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