See also https://bugs.kde.org/show_bug.cgi?id=319909

JIRA emails I receive are not threaded in message list. I tried all three methods: Perfect, Perfect and Reference, Perfect, Reference and Subject.

I think the reason is that these messages have both In-Reply-To and References headers filled, but there is never a parent with such ID. So they are in the same thread, but there is never a thread parent. JIRA developers say that it is set so, because you can start receiving mail during issue workflow, not from the begining, so they mark threads correctly with the same ID, but there is never parent.

I suspect that for some reason it never gets to Thread by Subject pass, so it would be matched perfectly. Subject is complicated as well.

Below are two samples of messages in same thread, but not threaded in KMail2.

Delivery-date: Thu, 16 May 2013 14:34:33 +0200
Date: Thu, 16 May 2013 14:33:37 +0200 (CEST)
From: "X" <jira@company.com>
To: X@company.com
Message-ID: <JIRA.139572.1368538114000.23735.1368707617359@jira.company.com>
In-Reply-To: <JIRA.139572.1368538114000@jira.company.com>
References: <JIRA.139572.1368538114000@jira.company.com>
MIME-Version: 1.0
Precedence: bulk
Auto-Submitted: auto-generated
X-JIRA-FingerPrint: fb2cdf5241d4431752db9b302372a195
Received-SPF: none client-ip=172.31.254.38; envelope-from=jira@company.com;
 helo=company.com
Subject: [Mailing.list] [company JIRA] (XXX-897) Error changing Permision
	profile for agents
X-BeenThere: mailing.list@company.com
X-Mailman-Version: 2.1.15
List-Id: <mailing.list.company.com>


Delivery-date: Thu, 16 May 2013 14:40:27 +0200
Date: Thu, 16 May 2013 14:39:37 +0200 (CEST)
From: "X" <jira@company.com>
To: mailing.list@company.com
Message-ID: <JIRA.139572.1368538114000.23744.1368707977266@jira.company.com>
In-Reply-To: <JIRA.139572.1368538114000@jira.company.com>
References: <JIRA.139572.1368538114000@jira.company.com>
MIME-Version: 1.0
Precedence: bulk
Auto-Submitted: auto-generated
X-JIRA-FingerPrint: fb2cdf5241d4431752db9b302372a195
Received-SPF: none client-ip=172.31.254.38; envelope-from=jira@company.com;
 helo=company.com
Subject: [Mailing.list] [company JIRA] (BUGS-897) Error changing Permision
	profile for agents
X-BeenThere: mailing.list@company.com
X-Mailman-Version: 2.1.15
List-Id: <mailing.list.company.com>


Reproducible: Always

Actual Results:  
Message list not threaded.

Expected Results:  
Message list threaded by References.