Created attachment 48282 [details]
Screen shot of a thread that should be joined, isn't

Version:           1.13.5 (using KDE 4.4.4) 
OS:                Linux

On some email lists I am on, KMail is unable to correctly thread messages no matter what I do.
I think I’ve found the issue- the ’references’ headers are just a copy of the ’mesage-id’ header (i.e. this message is a reply to itself). It should be a list (maybe even of just one item) of parent message id’s.
 
Example (partial header blocks from two messages, one is a reply to the other)>:
 
 Date: Tue, 22 Jun 2010 01:31:06 -0700
 X-Mailer: Chilkat Software Inc (http://www.chilkatsoft.com)
 Message-ID: <CRID_ab1b5dc52bbd4515a734d960e8a2c0e7@zzz.com>
 References: <CRID_ab1b5dc52bbd4515a734d960e8a2c0e7@zzz.com>
 Resent-Message-ID: <CRID_ab1b5dc52bbd4515a734d960e8a2c0e7@zzz.com>
 X-Priority: 3
 From: "David Hollr - General Discussions" <prforums@zzz.com>
 To: "Tom Chiverton" <tom.chiverton@f.com>
 Subject: RE: Open Type does not work with AS Files imported via SourcePath
 
 and
 
 Date: Tue, 22 Jun 2010 01:20:27 -0700
 X-Mailer: Chilkat Software Inc (http://www.chilkatsoft.com)
 Message-ID: <CRID_a2f23f2e46334e32bc7c3765d4d16f30@zzz.com>
 References: <CRID_a2f23f2e46334e32bc7c3765d4d16f30@zzz.com>
 Resent-Message-ID: <CRID_a2f23f2e46334e32bc7c3765d4d16f30@zzz.com>
 X-Priority: 3
 From: "Radishna Bhat - General Discussions" <prforums@zzz.com>
 To: "Tom Chiverton" <tom.chiverton@f.com>
 Subject: RE: Open Type does not work with AS Files imported via SourcePath

Reproducible: Always

Steps to Reproduce:
Join a mailing list that uses broken software (see x-mailer above).
Filter those messages into their own folder
Set the folder to use 'standard mailing list' aggregation.

Actual Results:  
All messages in a thread (as denoted by Re: Foo in subject) show as single-item threads, no matter what "view, message list, aggregation" option is used.

Expected Results:  
KMail should notice that the message refers only to itself, realise this must be broken, and fall back to aggregating by subject.

Being able to deselect 'perfect' and/or 'by references' but keep 'by subject' in the "customise message aggregation modes" settings would do the job quickly and easily. I.e. make these three items check boxes, not a drop down.