Attachments appear as winmail.dat

Attachments when sent from Outlook appear in Express or Eudora as .dat files when they should be another format e.g .doc

If an attachment is dragged and dropped on to a message rather than added as an attachment explicitly Outlook uses a different encoding scheme called TNEF.

Transport Neutral Encapsulation Format (TNEF) is a proprietary e-mail attachment format used by Microsoft Outlook and Microsoft Exchange Server. An attached file with TNEF encoding is most usually called winmail.dat or win.dat and has a MIME type of Application/MS-TNEF.

TNEF cannot be read by Express, Eudora, Outlook Express and many other clients.

Resolution

TNEF can be turned off in Outlook in three different ways:

1. Global: Changing your default mail format to Plain Text or HTML will help ensure that TNEF is not sent unless an Outlook feature needs it.
2. Per Message: If the message is a Rich Text Format (RTF) message, and you are using the Internet Mail Only (IMO) installation of Outlook, you can turn on or turn off TNEF for one message at a time.
3. Per Recipient: You can specify in the recipient's e-mail address to not send TNEF, so that a recipient always receives plain text versions of the message.

Global Change

• On the Tools menu, click Options, and then click the Mail Format tab.
• In the Send in this message format list, select Plain Text or HTML, and then click OK.

Per Message Change

• Open the message.
• On the File menu, click Properties.
• On the General tab, clear the Send in Microsoft Outlook Rich Text Format check box.

Per Recipient Change

• Create an Outlook contact for the recipient, if one does not already exist.
• Type the e-mail name of the recipient in the E-mail box of the contact.
• Select Send using plain text to turn off TNEF.

By selecting the check box, it ensures that TNEF will not be used for any messages to this recipient.

For more information please see the following Microsoft Knowledge Base Document on TNEF