What is an MS-TNEF email attachment?

The Microsoft email clients Exchange and Outlook use a format called Microsoft Transport Neutral Encapsulation Format (MS-TNEF) to send email messages in Rich Text Format (RTF). By default, Exchange and Outlook send email messages as if they will be received by other Microsoft email clients, extracting the formatting out of the original document and encoding it into a TNEF block. They then send the message in two parts: the first part is the text, while the second is the formatting.

If a Microsoft email client receives the message, it reassembles the two parts into a complete RTF document. However, with a non-Microsoft email client, the formatting part of the message will usually appear as an attached file named something like application/ms-tnef. If you are using a non-Microsoft email client, the text of the message should appear normal. However, you will not see the extra formatting ordinarily added by Outlook or Exchange, and the attachment will be of no use.

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