&lt;html xmlns:v="urn:schemas-microsoft-com:vml" xmlns:o="urn:schemas-microsoft-com:office:office" xmlns:w="urn:schemas-microsoft-com:office:word" xmlns:m="http://schemas.microsoft.com/office/2004/12/omml" xmlns="http://www.w3.org/TR/REC-html40"&gt;&lt;head&gt;&lt;meta http-equiv=Content-Type content="text/html; charset=us-ascii"&gt;&lt;meta name=Generator content="Microsoft Word 12 (filtered medium)"&gt;&lt;style&gt;&lt;/style&gt;&lt;/head&gt;&lt;body lang=NL link=blue vlink=purple&gt;<div><p><span>Last week I studied many Windows-Mail User Agents with the conversation threading feature.&lt;o:p&gt;&lt;/o:p&gt;</span></p><p><span>None of them (SUP, mutt-kz(notmuch), Outlook 2010, Thunderbird with conversation thread plug in, Postbox, Evolution) could cope with the following case:&lt;o:p&gt;&lt;/o:p&gt;</span></p><p><span>&lt;o:p&gt;&nbsp;&lt;/o:p&gt;</span></p><p><span>In our e-mail-discussions people often choose &#8216;reply-all&#8217; to construct a new message with the same reciepients.&lt;o:p&gt;&lt;/o:p&gt;</span></p><p><span>They clear the body and the subject, but the hidden References: and In-reply-To: stay and should be cleared as well.&lt;o:p&gt;&lt;/o:p&gt;</span></p><p><span>Result is that this new subject drowns in an old conversation-thread-drilldown and this unpredictable behavior makes conversation threading useless.&lt;o:p&gt;&lt;/o:p&gt;</span></p><p><span>This weekend I went analyzing the notmuch-source to find where I could put a fix best.&lt;o:p&gt;&lt;/o:p&gt;</span></p><p><span>&lt;o:p&gt;&nbsp;&lt;/o:p&gt;</span></p><p><span>I think of a fix that indexes the first dates of (stripped) subject-changes within threads, and with each first (stripped) subject change check the body on quotes of previous messages. If there is no quote to referenced mails then drop the reference and assign a new thread_id_ to the (stripped) subject.&lt;o:p&gt;&lt;/o:p&gt;</span></p><p><span>&lt;o:p&gt;&nbsp;&lt;/o:p&gt;</span></p><p><span>After two days of studying I think the best place with the least interference with existing code is between &#8216;notmuch new&#8217; and starting the MUA. Then the threads are in place in XAPIAN, and new thread_id_&#8217;s can be inserted.&lt;o:p&gt;&lt;/o:p&gt;</span></p><p><span>&lt;o:p&gt;&nbsp;&lt;/o:p&gt;</span></p><p><span>Am I right?&lt;o:p&gt;&lt;/o:p&gt;</span></p><p><span>&lt;o:p&gt;&nbsp;&lt;/o:p&gt;</span></p></div>&lt;/body&gt;&lt;/html&gt;