我们的网站主要通过 PHP 发送大量不同的电子邮件。它们曾经在电子邮件程序中分组/线程化，现在它们不是我的理解，没有任何改变。下面是应该分组的两封电子邮件，因为它们包含相同的 In-Reply-To，但它们不在 Outlook 内部或 iPhone 上。两封邮件都显示
In-Reply-To: <sw-ordernotify-event-14066@ourdomain.com>
以前总是将它们分组。谁能看到为什么这些电子邮件没有被分组？
电子邮件 1
Delivered-To: MYEMAILADDRESS@ourdomain.com
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Return-Path: <no-reply@ourdomain.com>
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Fri, 10 Mar 2017 19:24:02 -0800 (PST)
Message-ID: <58c36dd2.15106b0a.33d42.838e@mx.google.com>
Date: Fri, 10 Mar 2017 19:24:02 -0800 (PST)
From: "OurDomain.com" <no-reply@ourdomain.com>
X-Google-Original-From: "OurDomain.com" <instantordernotification@ourdomain.com>
MIME-Version: 1.0
Content-Type: multipart/alternative; boundary="=_a3d730346de879d36ef3d83fa85aa6fa"
Subject: OurDomain.com: Order Notification for Famous Dex Live in Concert
In-Reply-To: <sw-ordernotify-event-14066@ourdomain.com>
Cc: <MYEMAILADDRESS@ourdomain.com>
To: <MYOTHEREMAILADDRESS@yahoo.com>
电子邮件 2
Delivered-To: MYEMAILADDRESS@ourdomain.com
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Return-Path: <no-reply@ourdomain.com>
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for <MYEMAILADDRESS@ourdomain.com>
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Message-ID: <58c36dd4.e4886b0a.95c82.821a@mx.google.com>
Date: Fri, 10 Mar 2017 19:24:04 -0800 (PST)
From: "OurDomain.com" <no-reply@ourdomain.com>
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MIME-Version: 1.0
Content-Type: multipart/alternative; boundary="=_316d1e71935372135c22d7148ac225ee"
Subject: OurDomain.com: Order Notification for Famous Dex Live in Concert
In-Reply-To: <sw-ordernotify-event-14066@ourdomain.com>
Cc: <MYEMAILADDRESS@ourdomain.com>
To: <MYOTHEREMAILADDRESS@yahoo.com>