当提供相同的主题行不起作用时？
我想发送 2 封电子邮件并让它们出现在一个对话中。可悲的是，第二个总是开始新的对话。如何避免这种情况？
两封邮件都直接发送到my.address@gmail.com
两者都有几乎相同的标题和相同的 UTF-8 编码主题：
Delivered-To: my.account@gmail.com
Received: by ...
Return-Path: ...
Received: ...
Received-SPF: pass ...
Authentication-Results: mx.google.com; spf=pass ...
Received: from mm_www by ...
To: my.account@gmail.com
Subject: =?UTF-8?...=?=
From: notification@my.domain
Content-Type: text/plain; charset=UTF-8; format=flowed
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
X-Mailer: PHP
Message-Id: ... (Different for both messages. Should I add my own id?)
Date: ...
有什么线索吗？