Outlook has information that is sent along with a message that, AFAIK, is outside of the SMTP standard. For example, you can mark a message with a due date or a to-do flag. How is that information embedded in the email when one Outlook client sends it over the open Internet to another Outlook client? I don’t see anything weird if I look at it on a web mail client. How does this meta-data get hidden from ordinary mail clients but is visible to another Outlook client?
The same question applies to meeting invitations.
(This would not be so mysterious if messages were sent strictly by Microsoft Exchange, but I see the same thing using SMTP.)
The messages come through as multipart MIME emails, with the extra features encoded in a binary segment of the email. I’ve just sent myself a message with voting enabled, and yes, Outlook just sticks a binary MIME segment on the end of the email, with stuff that only Outlook (or anyone who knows Microsoft’s mail format) can decode. I’m not massively familiar with MIME, but loosely speaking it’s a packaging standard that allows the sending of binary and other types of files using the exclusively 7-bit ASCII SMTP standard. Picture attachments, alternative character sets; all this stuff depends on MIME to tell the email client how the message is split up, and how the bits should be interpreted. Here’s yer obligatory Wiki link (from which you can find the original RFCs if you’re in a masochistic mood… :)).
Here’s what the whole email looks like in plain text; the Outlook bit is the horrible mess at the end.
From <snip> Fri Feb 01 19:56:14 2008
Return-path: <snip>
Envelope-to: <snip>
Delivery-date: Fri, 01 Feb 2008 19:56:14 +0000
<snipped routing data>
From: "Me"
To: "'Me'"
Subject: test
Date: Fri, 1 Feb 2008 19:56:20 -0000
Message-ID: <!&!AAAAAAAAAAAYAAAAAAAAAG9Us7R6YMFPr4c1BQ5RltYihQAAEAAAAEF/ZaddFDNItOap9ghzkrEBAAAAAA==>
MIME-Version: 1.0
Content-Type: multipart/mixed;
boundary="----=_NextPart_000_001E_01C8650C.8395E1B0"
X-Mailer: Microsoft Office Outlook 12.0
Thread-Index: AchlDINtjvE/NvgMSIqKXeJ3tMHqZQ==
X-MS-TNEF-Correlator: 000000006F54B3B47A60C14FAF8735050E5196D604403100
Content-Language: en-gb
X-BrightMail-Verdict: VlJEQwAAAAIAAAABAAAAAAAAAAEAAAAAAAAAAWluYm94AHNwYzAzQGRvYy5pYy5hYy51awA=
X-BrightMail-Tracker: AAAAAA==
X-Spam-Flag: NO
X-Spam-Level:
Status: O
X-Status:
X-Keywords:
X-UID: 872
This is a multipart message in MIME format.
------=_NextPart_000_001E_01C8650C.8395E1B0
Content-Type: text/plain;
charset="iso-8859-1"
Content-Transfer-Encoding: 7bit
fish
------=_NextPart_000_001E_01C8650C.8395E1B0
Content-Type: application/ms-tnef;
name="winmail.dat"
Content-Transfer-Encoding: base64
Content-Disposition: attachment;
filename="winmail.dat"
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------=_NextPart_000_001E_01C8650C.8395E1B0--