How email message formats affect Internet email messages in Outlook

Summary

Both Microsoft Outlook and the Microsoft Exchange Client sometimes use a special method to package information for sending messages across the Internet. This method is technically known as Transport Neutral Encapsulation Format (TNEF). Technical information on TNEF is available at the following MSDN website:

http://msdn.microsoft.com

More information

The use of TNEF is typically affected by settings in Outlook that are known as Microsoft Outlook Rich Text Format (RTF). RTF and TNEF are closely related, but not the same. A TNEF-encoded message contains a plain text version of the message, and a binary attachment that "packages" various other parts of the original message. In most cases, the binary attachment is named Winmail.dat, and may include the following information:

The formatted text version of the message (for example, font information and colors).
OLE objects (for example, embedded pictures and embedded Microsoft Office documents).
Special Outlook features (for example, custom forms, voting buttons, and meeting requests).
Regular file attachments that were added to the original message.

In addition to the previously listed information, the path of your personal folders (.pst) file and your log on name are embedded in the Winmail.dat file. Although this data is not explicitly exposed to the recipient, if the recipient opens the Winmail.dat file for editing in a binary or text editor, they can see the path and log on name. Be aware that the password information is not revealed. To make sure that the path of your .pst file or your log on name is not included in the Winmail.dat attachment, follow the steps in this article to send messages that do not include the Winmail.dat file. Some Outlook features require TNEF encoding to be understood correctly by an Internet email recipient who also uses Outlook. For example, when you send a message that has Voting buttons to a recipient over the Internet, if TNEF is not enabled for that recipient, the Voting buttons are not received. Or, for sending messages that have regular file attachments, TNEF is not needed. If you are sending messages that have file attachments to a recipient who does not use Outlook or the Exchange Client, we recommend that you manually choose to use an email format which does not require TNEF (such as HTML or plain text). If a message is sent without TNEF, the recipient can view and save the attachments as expected.

Sending and receiving concerns

When an email client that does not understand TNEF receives a message that contains TNEF information, the following are the two common results:

The plain text version of the message is received and it contains an attachment named Winmail.dat. The Winmail.dat attachment has no useful information when it is opened because it is in the TNEF format.
The plain text version of the message is received and it contains an attachment that has a generic name such as ATT00008.dat or ATT00005.eml. In this case, the client is unable to recognize either the TNEF part of the message or the Winmail.dat file name. Therefore, a file name is created to hold the TNEF information.

In addition to the receiving client, it is common for an email server to strip out TNEF information from messages as it delivers them. If a server option to remove TNEF is turned on, clients will always receive a plain text version of the message. Exchange Server is an example of an e-mail server program that has the option to remove TNEF from messages.

Message encoding

The Internet standards for encoding messages such as Multipart Internet Mail Extensions (MIME) and UUENCODE are used independently of TNEF. TNEF can exist in a MIME-encoded message as a MIME body part of type "application/ms-tnef," or in a UUENCODED message as an attachment named Winmail.dat. The sent TNEF encoding must be understood by the receiving client to correctly display the encapsulated information.

How to control TNEF in messages

You can control TNEF by using one of three methods:

Global - If you change your default email format to plain text or HTML, it helps make sure that TNEF is not sent unless an Outlook feature needs it.
Per Recipient - You can specify in the recipient's email address not to send TNEF so that a recipient always receives plain text versions of the message.
Per Message - When you compose a new message, or reply to a received message.

Method 1. How to make a global change for TNEF

For Microsoft Outlook 2010 and later versions: To turn off TNEF, follow these steps:

Click the File tab, click Options, and then click Mail.
In the Compose in this message format list, click Plain Text or HTML, and then click OK.

To send messages in TNEF, follow these steps:

Click the File tab, click Options, and then click Mail.

Method 2: How to make a Per Recipient Change for TNEF

For Outlook 2010: In the Contacts Folder To turn off TNEF, follow these steps:

Open the recipient's record in the Contacts folder.
Double-click the recipient's email address.
The Contact Card appears. Click View more options for interacting with this person, and then click Outlook properties.
Select Send Plain Text only in the Internet Format list.

To send in TNEF, follow these steps:

Open the recipient's record in the Contacts folder.
Double-click the recipient's email address.
The Contact Card appears. Click View more options for interacting with this person, and then click Outlook properties.
Click Send Using Outlook Rich Text Format in the Internet Format box.

Method 3: How to make a per-message change for TNEF

For Outlook 2010 and later versions: To turn off TNEF, follow these steps:

Open a new mail message, or click Reply on a received message.
On the Format Text tab, click HTML or Plain Text.

To turn on TNEF, follow these steps:

Open a new mail message, or click Reply on a received message.
On the Format Text tab, click Rich Text.

Common Scenarios

Problem: Recipient receives the Winmail.dat attachment. Solution: Turn TNEF off (either for the recipient, or globally).
Problem: Recipient receives the ATT00001.DAT attachment. Solution: Turn TNEF off (either for the recipient, or globally).
Problem: Recipient does not receive regular file attachments. Solution: Turn TNEF off (either for the recipient, or globally).
Problem: Recipient does not have Voting buttons in Outlook. Solution: Turn TNEF on for the recipient.
Problem: Recipient receives meeting requests as regular messages. Solution: Turn TNEF on for the recipient.
Problem: Recipient does not receive custom form information. Solution: Turn TNEF on for the recipient.
Problem: Recipient does not receive formatted message text. Solution: Turn TNEF on for the recipient.