Hi
I'm sending from a utility multiple related e-mails with different subjects.
As these e-mails are related and there is one "main" e-mail and several "sub" e-mails, I want them to be shown in Outlook as conversation if the view option is enabled.
The sub-e-mails have the In-Reply-To and References headers properly set, yet this doesn't cause anything in Outlook.
So I searched around and found the Thread-Index header which Outlook is said to use for defining a conversation, so I tried generating those. Unfortunately this does only work partly. I tried sending the same 5 e-mails, (1 main, 4 sub) multiple times. Each time all 5 messages have the same Thread-Index, only the Message-IDs and timestamps are differently, the remaining is the same.
Yet it does not display properly and reliably in Outlook. Sometimes all 5 e-mails are in one conversation, sometimes 2 or 3 of them (not consistently the same) are in a conversation and the rest is displayed separately, often all 5 are displayed separately.
The Python code that generates the Thread-Index value is
thread_index = b'\x01\x00\x00\x12\x34\x56' + uuid.uuid4().bytes
return base64.standard_b64encode(thread_index).decode('ascii')
which should be conform to the documentation I found at https://msdn.microsoft.com/en-us/library/ee202481%28v=exchg.80%29 and https://docs.microsoft.com/en-us/office/client-developer/outlook/mapi/tracking-conversations as far as I can tell. (I also tried with the actual time encoded into the FILETIME part, but it didn't make any difference)
Can someone shed some light onto this issue and why it works so unreliably or how to make it working reliably?
