How does the email header field 'thread-index' work?

Question

I was wondering if anyone knew how the thread-index field in email headers work?

Here's a simple chain of emails thread indexes that I messaged myself with.

Email 1 Thread-Index: AcqvbpKt7QRrdlwaRBKmERImIT9IDg== Email 2 Thread-Index: AcqvbpjOf+21hsPgR4qZeVu9O988Eg== Email 3 Thread-Index: Acqvbp3C811djHLbQ9eTGDmyBL925w== Email 4 Thread-Index: AcqvbqMuifoc5OztR7ei1BLNqFSVvw== Email 5 Thread-Index: AcqvbqfdWWuz4UwLS7arQJX7/XeUvg== 

I can't seem to say with certainty how I can link these emails together. Normally, I would use the in-reply-to field or references field, but I recently found that Blackberrys do NOT include these fields. The only include Thread-Index field.

Answer 1

They are base64 encoded Conversation Index values. No need to reverse engineer them as they are documented by Microsoft on e.g. http://msdn.microsoft.com/en-us/library/ms528174(v=exchg.10).aspx and more detailed on http://msdn.microsoft.com/en-us/library/ee202481(v=exchg.80).aspx

Seemingly the indexes in your example doesn't represent the same conversation, which probably means that the software that sent the mails wasn't able to link them together.

EDIT: Unfortunately I don't have enough reputation to add a comment, but adamo is right that it contains a timestamp - a somewhat esoteric encoded partial FILETIME. But it also contains a GUID, so it is pretty much guarenteed to be unique for that mail (of course the same mail can exist in multiple copies).