Very delayed mail delivery. What happened?

Question

I have an online form that deliver email to my gmail account. it had been appearing to work normally for a long time, years, until 6 Sep 2011 when suddenly 250+ emails arrived in my inbox at one go. But looking at the header, these were sent between June and August. Can anyone decipher from the header what happened? Otherwise what mechanism might have cause this?

Delivered-To: *******@gmail.com
Received: by 10.223.96.8 with SMTP id f8cs74668fan;
        Mon, 5 Sep 2011 23:27:50 -0700 (PDT)
Received: by 10.91.110.8 with SMTP id n8mr3292513agm.37.1315290469625;
        Mon, 05 Sep 2011 23:27:49 -0700 (PDT)
Return-Path: <httpd@web7.dh.com.sg>
Received: from web7.dh.com.sg ([203.117.141.130])
        by mx.google.com with ESMTPS id n15si3643530anb.191.2011.09.05.23.27.47
        (version=TLSv1/SSLv3 cipher=OTHER);
        Mon, 05 Sep 2011 23:27:48 -0700 (PDT)
Received-SPF: neutral (google.com: 203.117.141.130 is neither permitted nor denied by best guess record for domain of httpd@web7.dh.com.sg) client-ip=203.117.141.130;
Authentication-Results: mx.google.com; spf=neutral (google.com: 203.117.141.130 is neither permitted nor denied by best guess record for domain of httpd@web7.dh.com.sg) smtp.mail=httpd@web7.dh.com.sg
Received: from web7.dh.com.sg (localhost [127.0.0.1])
    by web7.dh.com.sg (8.13.1/8.13.1) with ESMTP id p866RV6P031284
    for <*******@gmail.com>; Tue, 6 Sep 2011 14:27:45 +0800
Received: (from httpd@localhost)
    by web7.dh.com.sg (8.13.1/8.13.1/Submit) id p5U9UX4N030130;
    Thu, 30 Jun 2011 17:30:33 +0800
Date: Thu, 30 Jun 2011 17:30:33 +0800
Message-Id: <201106300930.p5U9UX4N030130@web7.dh.com.sg>

Answer 1

Received: (from httpd@localhost)
    by web7.dh.com.sg (8.13.1/8.13.1/Submit) id p5U9UX4N030130;
    Thu, 30 Jun 2011 17:30:33 +0800

is the culprit. It received the mail in June and didn't forward. For whatever reason. Then on September 6th it delivered the delayed mail to

Received: from web7.dh.com.sg (localhost [127.0.0.1])
    by web7.dh.com.sg (8.13.1/8.13.1) with ESMTP id p866RV6P031284
    for <*******@gmail.com>; Tue, 6 Sep 2011 14:27:45 +0800

So either the first one was unable to deliver or the second one was unable to receive. There are many reasons so this could have happened.

1. Network error
2. Storage/Quota exceeded on second one
3. SMTP service on first one failed
4. SMTP service on second was down
5. Software failure
6. ...

Answer 2

For the message that you've provided the headers for, it looks as though it spent a couple of months hanging out at one of DifferentHost's mail queues; my guess would be a misconfigured server at DH that wasn't forwarding mail, or dropping it. Then someone sorts it out, and all the mail gets processed, arriving at your inbox a couple of months after it was sent.

A similar thing happened to me when I upgraded our GroupWise server from 5.5 to 6 (it was a while ago, stop looking at me like that); a couple of emails had apparently been stuck in our queue as well, and we surprised a couple of users with emails from about 6 months ago. Nothing important, thankfully...