This article presents some basics on Transport Neutral Encapsulation Format (TNEF).

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Transport Neutral Encapsulation Format (TNEF) is a Microsoft proprietary method for encoding Rich Text Format (RTF) messages sent across the Internet. A TNEF-encoded message contains a plain text version of the message, an attachment that contains the RTF version, and any OLE objects such as pictures or other attachments that were sent with the message. Some e-mail clients do not understand TNEF encoding, and so it should only be used if you are certain the message will be received by someone using Windows Messaging, Microsoft Exchange Client, or Outlook.

When a message containing TNEF information is received by a mail client that does not understand TNEF, an attachment is created with the name Winmail.dat. In some cases, the mail server will look for and strip out the TNEF and Winmail.dat information.

There are servers that can be configured to look for TNEF and winmail.dat and strip it out, but the usual symptom in that case is that the servers completely remove the Winmail.dat attachment.

For additional information about Winmail.dat, please see the following article in the Microsoft Knowledge Base: NOTE: The information in the article mentioned, applies if the sender is using Outlook in Corporate/Workgroup (CW) mode. The Internet Mail Only (IMO) mode works quite differently with regards to TNEF.

•	Some features require the message be sent in RTF format. For example, in CW mode, to get meeting requests to work (i.e. send TNEF), you need to double-click the recipient's email address and select Send to this recipient in Microsoft rich text format. This is not the same thing as composing a new message with your default message format set to Microsoft Outlook Rich Text (RTF). In IMO mode, there is no extra step to making a meeting request work. It will always be sent using TNEF.
•	If you get Winmail.dat in your e-mail, it means that your server doesn't work with TNEF. What that usually means is that the person reading the email is using a client that does not understand TNEF. If the recipient is using Outlook 98 or Outlook 2000 IMO, and sees winmail.dat, then it's possible that something damaged the message.
•	RTF enables TNEF. In Internet Mail Only mode, if you send an RTF message, it will be sent using TNEF. In Corporate or Workgroup mode, if you send an RTF message, it could be stripped to plain text depending on several settings.

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Troubleshooting Questions

•	Can you confirm what client is used to receive the meeting requests?
•	The customer says it worked with Outlook 97 - were they using the Internet E-mail Service or was there an Exchange Server somewhere in the mix?
•	What path does the meeting request take: Outlook Internet Mail Only mode submits an RTF message via Simple Mail Transport Protocol (SMTP), to a Lotus SMTP server and the message is delivered to a mailbox on that same Lotus server, and downloaded via Post Office Protocol (POP) with what client?

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For additional information about Transport Neutral Encapsulation Format, please see the following articles in the Microsoft Knowledge Base:

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