MS Outlook Uses Non-Standard Format (TNEF)

The basic problem is that in certain cases Microsoft Outlook uses a nonstandard extra packaging mechanism called "ms-tnef" or "tnef" when it sends email; typically when it sends attachments. Outlook is suppose to simply use the industry standards (such as MIME and HTML) directly for attachments. The full name of the format is "Transport Neutral Encapsulation Format", but that is a misleading name... it may be neutral on transport, but it obstructs reception. Almost no other email reader can read this nonstandard format. Email clients that cannot (currently) read this format include Lotus Notes, Thunderbird / Netscape Mail, and Eudora. In fact, even Microsoft's own Outlook Express can't read this format!

If your e-mail program receives a TNEF file and doesn't understand it, instead of seeing the e-mail and/or attachment, you may only see an attachment named "Winmail.dat", "Part 1.2", or something else ending in ".dat" or ".eml".

How To Fix Outlook

If you use Microsoft Outlook, and want to send attachments outside your office, you have to reconfigure Outlook so that other people can read the files you send :-(.

  • On the "Tools" menu, click "Options"
    tnef01:

  • Then click the "Mail Format" tab, and then the "Internet Format" button.
    tnef02:

  • Set "When sending Outlook Rich Text messages to Internet ..." to either "Convert to HTML format" or "Convert to Plain Text format".
    tnef03:

    If you prefer, in all versions of Outlook, you can disable TNEF completely:
    1. On the "Tools" menu, click "Options", and then click the "Mail Format" tab.
    2. In the "Compose in this message format" list, click "Plain Text" or "HTML", and then lick "OK".

    One oddity: don't send a calendar entry / meeting invitation and an attachment in the same message; some versions of Outlook will send TNEF in this case, even if you've told it not to.
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