How to make GMail continue a conversation? -

same subject line is not working?

I want to send you 2 emails and make them appear in a conversation Regrettably, the second one always starts a new conversation to avoid this?

Both mails are sent directly to my.address@gmail.com

Both have encoded almost the same header and the same UTF-8 Given topic:

  Distributor: my.account@gmail.com Received: By ... Return-Path: ... Received ... ... Received-SPF: near .. . Authentication result: mx.google.com; Spf = pass ... Received: by mm_www ... from: my.account@gmail.com Topic: =? UTF-8? ... =? = From: Notification @ my.domain Content-Type: Text / Plane; Charset = UTF-8; Format = Flow MIME-version: 1.0 Content-Transfer-Encoding: 8 bit X-Mailer: PHP Message-ID: ... (Separate for both messages. Should I add my own ID?) Date: ...  

Any clues?

Yes, message ID uses it to determine what is important here customer flow.