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

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==
Em开发者_运维百科ail 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.

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).

There's a good analysis of how exactly this non-standard "Thread-Index" header appears to be used, in this post and links therefrom, including this pdf (a paper presented at the CEAS 2006 conference) and this follow-up, which includes a comment on the issue from the evolution source code (which seems to reflect substantial reverse-engineering of this undocumented header).

Executive summary: essentially, the author eventually gives up on using this header and recommends and shows a different approach, which is also implemented in the c-client library, part of the UW IMAP Toolkit open source package (which is not for IMAP only -- don't let the name fool you, it also works for POP, NNTP, local mailboxes, &c).

I wouldn't be surprised if there are mail clients out there which would not be able to link Blackberry's mails to their threads. The Thread-Index header appears to be a Microsoft extension.

Either way, Novell Evolution implements this. Take a look at this short description of how they do it, or this piece of code that finds the thread parent of a given message.

I assume that, because the lengths of the Thread-Index headers in your example are all the same, these messages were all thread starts? Strange that they're only 22-bytes, though I suppose you could try applying the 5-bytes-per-message rule to them and see if it works for you.

If you are interested in parsing the Thread-Index in C# please take a look at this post

http://forum.rebex.net/questions/3841/how-to-interprete-thread-index-header

The snippet you will find there will let you parse the Thread-Index and retrieve the Thread GUID and message DateTime. There is a problem however, it does not work for all Thread-Indexes out there. Question is why do some Thread-Indexes generate invalid DateTime and what to do to support all of them???