Transport Neutral Encapsulation Format - Overview

Overview

Some TNEF files contain information used by only Outlook to generate a richly formatted view of the message, such as embedded (OLE) documents or Outlook-specific features such as forms, voting buttons, and meeting requests. Other TNEF files may contain files which have been attached to an e-mail message.

Within the Outlook e-mail client TNEF encoding cannot be explicitly enabled or disabled (except via a registry setting). Selecting RTF as the format for sending an e-mail implicitly enables TNEF encoding, using it in preference to the more common and widely compatible MIME standard. When sending plain text or HTML format messages, some versions of Outlook (apparently including Outlook 2000) prefer MIME, but may still use TNEF under some circumstances (for example, if an Outlook feature requires it).

TNEF attachments can contain security-sensitive information such as user login name and file paths, from which access controls could possibly be inferred.