I have a piece of software that is sending multiple e-mails.
The first one has the header

Message-ID: <153495492389.29001.486258560116169081@server.example.com>

The second and all following ones have the headers

In-Reply-To: <153495492389.29001.486258560116169081@server.example.com>
References: <153495492389.29001.486258560116169081@server.example.com>

But it seems that this is not enough for Outlook to display those messages as conversation, even if showing conversations is enabled.

Can someone enlighten me what is necessary so that the mails can properly shown as conversation in Outlook?

  • Never seen anything official but my observation is that the subject line must match (prepended FW: and RE: don't count). Aug 22 '18 at 19:06
  • That is my observation also. From, To, Cc, etc., addresses don't count either. Aug 22 '18 at 21:35
  • Thanks, but that is not true, thank god. I frist thought you are right as with the same subject it worked. But that is only used as fallback. Bugzilla to the rescue: bugzilla.mozilla.org/show_bug.cgi?id=411601. You need to have the same UUID in the Thread-Index header, then they are grouped no matter whether the subject is the same or not, but the has to follow the format nevertheless. Any arbitrary value will not work, as I tested this before I wrote here already.
    – Vampire
    Aug 22 '18 at 22:57

Bugzilla to the rescue, they found out what to do a couple of years ago already at https://bugzilla.mozilla.org/show_bug.cgi?id=411601.

The key is the Thread-Index header. It is a Base64 encoded sequence of some bytes and has amongst other fields a UUID field which is significant for the grouping.

The exact format is defined at https://msdn.microsoft.com/en-us/library/ee202481%28v=exchg.80%29


For Internet mail accounts and newer versions of Exchange accounts, the Conversation group feature uses a Conversation ID filed to identify if a message belongs to a conversation. You can use the MFCMAPI tool to view the Conversation ID filed: enter image description here

Reference: Conversations View (Show Messages from Other Folders)

  • And how is the Conversation ID different to the Thread-Index?
    – Vampire
    Aug 23 '18 at 8:50

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