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TNEF (Chapter 1): Basics

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I’ve worked quite a few cases recently regarding problems some folks have had either reading or composing TNEF content. I’ve learned quite a bit myself as a result, and I thought I’d share. I decided I would do a series of blog posts on the topic and hopefully save some of you the time I spent learning all this.

So, being the first post on the topic, I suppose now would be a good time for a review on just what TNEF is and how it’s structured.

TNEF stands for Transport-Neutral Encapsulation Format. If you use Outlook or any other MAPI client as your mail client, you may know that MAPI is a protocol for communication between the client and the mailbox server. MAPI defines a set of interfaces which the client can use to work with the data in the mailbox. The MAPI structure for the data is hierarchical with messages being contained in containers, which themselves can have a parent container, all the way up to the root of the store. MAPI also defines a set of properties understood by the client and, in some cases, the server. If all mail sent could stay on this one server and only go between clients on this one system, this would be all we need; but we know that’s not the case. A very large quantity of e-mail is sent over the internet to foreign systems every day. The vast majority of those use an industry-standard protocol called SMTP (Simple Mail Transfer Protocol) to send messages in an industry-standard format called MIME (Multipurpose Internet Mail Extensions). MIME is composed of body parts, which can in turn be composed of additional body parts themselves. MIME also allows you to add headers to each of the body parts which allow you to describe the content of that body part. So one body part may be a Word Document attachment, so the MIME headers on that body part would contain the MIME type such as application/doc and the transfer type, such as base64. The content of that body part would then contain a base64 encoding of the document. The headers for the root body part contain information such as the subject of the message, the sender and recipient information, etc.

As a message makes its way through transport from one person’s email client to another person’s, it encounters many “hops” (brief stops at SMTP servers in the routing path) which have the opportunity to modify the headers. They do this in order to track the path the message took or to flag it as SPAM, verify the sender address, etc. So there’s a chance the headers you specify when you send the message won’t be the same as when the message arrives at the destination. The headers also only support text values. One of the problems discovered early on about the MIME format was that it has no concept of “rich text.”

In early versions of Outlook, users wanted the ability to send and receive email that contained rich text bodies. Microsoft devised a plan to create an attachment to the messages it was sending that would have a certain content-type and would come to have a well-known name, “winmail.dat”. This attachment would contain an encapsulation of the MAPI properties that could represent this rich body that would work across any transport and be readable by any system that supported MIME.

The original structure just supported a very simple structure that was basically Name/size/value. These were called “attributes” and the names of these attributes are still prefixed by “att.” Many of the attribute names can be seen here: https://msdn.microsoft.com/en-us/library/cc765736.aspx. The most important of these attributes for the purposes of our discussion will be the attMAPIProps. This attribute contains a list of MAPI properties that the receiving system can set on the message once it has converted the other MIME parts into their MAPI format. Some of the TNEF attributes can be directly translated into MAPI properties as defined by the link earlier in this paragraph, but there is not a 1-1 mapping between TNEF attributes and MAPI properties – hence the attMAPIProps attribute. Attachments and recipient data can also be encoded into the TNEF structure, which we’ll examine more later.

MSDN documents the general structure of TNEF but it’s hard to understand. Last year, when Exchange decided to be among those systems that elected to publicly document their protocols, they created [MS-OXTNEF].pdf, which documents very clearly the structure of the TNEF data and how to parse it. Don’t get too nervous, though. I have parsed a 3MB TNEF blob manually myself, but in Exchange 2007, we provide managed code interfaces to allow you to read (or write) this data very easily. In subsequent posts, I’ll dive more into the structure and into the managed classes, as well as the legacy MAPI ITnef interfaces, and more into problems you may experience in developing TNEF-enabled applications.

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