Lloyd,
I see this happening with mails that have NO date header at all. Some mail clients at the receiving end seem to work around this, others don't.
I normally use Thunderbird so Thunderbird will show those mails with a date 1.1.1970 but Outlook Express will show the same mail (easy to do this with an imap account) with a "proper" date/time in it's inbox. It must make something up for that and use another date/time information from the mail headers?
Example for missing date headers (some info crossed out):
Received: (qmail 7380 invoked by alias); 14 Mar 2005 03:06:50 -0000
Delivered-To: alias-localdelivery-michael@tw.co.nz
Received: (qmail 7363 invoked from network); 14 Mar 2005 03:06:49 -0000
X-Virus-Scanned: by amavis-ng-0.1.6.4-03dc on tga1.tw.co.nz
Received: from xxx.co.nz (203-x-x-x.dsl.clear.net.nz [x.x.x.x])
by tga1.tw.co.nz ([192.168.1.2])
with SMTP via TCP; 14 Mar 2005 03:05:32 -0000
Received: (qmail 6996 invoked from network); 14 Mar 2005 01:41:14 -0000
Received: from pc-00028.x.co.nz (HELO PC00028) (192.168.1.28)
by x.x.co.nz (192.168.1.2) with SMTP; 14 Mar 2005 01:41:14 -0000
Message-ID: <001b01c52836$dc5c51f0$1c01a8c0@PC00028>
From: "A X" <a@x.co.nz>
To: "\"Recipients, Newsletter\"" <info@x.co.nz>
Subject: Newsletter - February 2005
MIME-Version: 1.0
Content-Type: multipart/mixed;
boundary="----=_NextPart_000_0018_01C528A3.D3211C70"
X-Mailer: Microsoft Outlook Express 6.00.2800.1437
X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2800.1441
X-BitDefender-SpamStamp: 1.1.3
044000040111CAAAAAAAAAAAAAAAAAAAAABAAAAAAQ
X-BitDefender-Scanner: Clean, Agent: BitDefender Qmail 1.6.1 on
tga1.tw.co.nz
X-BitDefender-Spam: No (0)
This is a multi-part message in MIME format.