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I don't know why, but every now and then I receive mails that should be a reply to or a forward on a mail that I already have. For some reason that new incoming mail isn't always connected to the mail that I already have. So I don't have a complete tree-structure of the original mail.

Currently I export the mail, manually add the proper header information, import the mail again and delete its previous version that came in from the mail server. Kind of cumbersome...

Is there a plug-in where I can select the two mails and it will connect them (the newer one becoming a reply or forward of the older mail)? I checked Thunderbird's plug-in page but couldn't find one.

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@RedGrittyBrick and that should help me how? –  sjngm Aug 2 '12 at 8:17
    
Fair cop, I occasionally misuse Comments (which are not Answers). • You say Thunderbird's threading is buggy, • JWZ's article suggests a solution (if we assume that Mozilla continued with the 4.0 algorithm). • Mozilla have a bug reporting system, • ??? , • Profit. –  RedGrittyBrick Aug 2 '12 at 8:40
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1 Answer

No such plugin?

If there isn't a plugin on Thunderbird's plugin page that addresses this, It may be because no such plugin yet exists.

Why not?

Such a plugin might have to change the data stored for messages - for example to add a reference to a message header so that the "connection" between messages is remembered for next time you start Thunderbird and open that folder. There are indications† that Thunderbird might still use Mork for some data structures relating to threading. Mork has a reputation for being unreasonably obscure and difficult. The Mozilla folk have been struggling for decades to rid their codebase of Mork, they succeeded with Firefox. The use of Mork suggests that writing a plugin to manipulate the data would be challenging. Of course this is largely speculation on my part.

nsMsgThreads are a thin wrapper around their underlying mork representation, keeping only keys/cached values in memory, with all important/mutable data being stored in mork.

What can cause disconnected threads?

Outlook (or at least some versions thereof)

  • Doesn't set the In-Reply-To or References headers (as it SHOULD)
  • Can insert unexpected prefixes in the Subject line (e.g. "AW:" instead of "RE:")

This combination makes it near impossible to thread such messages. There are Thunderbird bug-reports that discuss how to deal with this (maybe this specific problem is resolved now?)

It is possible that other mail clients exist that do not produce these headers in a useful form.

What are these headers?

RFC822 says

 4.6.2.  IN-REPLY-TO

         The contents of this field identify  previous  correspon-
    dence  which this message answers.  Note that if message iden-
    tifiers are used in this  field,  they  must  use  the  msg-id
    specification format.

 4.6.3.  REFERENCES

         The contents of this field identify other  correspondence
    which  this message references.  Note that if message identif-
    iers are used, they must use the msg-id specification format.

RFC2822 says

3.6.4. Identification fields

   Though optional, every message SHOULD have a "Message-ID:" field.
   Furthermore, reply messages SHOULD have "In-Reply-To:" and
   "References:" fields as appropriate, as described below.

...

   The
   "In-Reply-To:" field may be used to identify the message (or
   messages) to which the new message is a reply, while the
   "References:" field may be used to identify a "thread" of
   conversation.

What practical use is this answer?

Think of it as a placeholder until a better answer arrives.

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Thanks for extending your comment to this full answer (+1). Maybe I should give it a try and write an extension on my own... (never done that, lack of time) - I just wished that someone would point me to a "hidden" hive of extensions where my problem could find a perfect remedy... –  sjngm Aug 3 '12 at 5:27
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