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Why do recipients receive winmail.dat instead of the attachments I sent to them?


In Outlook 2003 or Outlook 2007, occasionally when you attempt to send one or more attachments to non-Central Exchange users (either on or off campus), some recipients report that the message includes an additional file called Winmail.dat. The recipient is unable to open this file, or alternately may be able to open the file in a text editor, but its contents are unreadable. This may not occur for all messages sent with attachments, but it usually occurs only with specific non-Exchange users. The problem is more likely to occur when using the Rich Text Format (RTF) to send messages, however may occur without expressly selecting RTF when composing new messages or replying to an existing message.

Problem Description

This problem occurs because the Winmail.dat file, which is a Transport Neutral Encoding Format (TNEF-Encoded) attachment, is used in Rich Text Format to preserve formatting that the sending client includes in the message, however the receiving client does not recognize the Winmail.dat file. In Outlook, the Winmail.dat file includes Rich Text Formatting (RTF) instructions. This type of formatting is used with the Microsoft Outlook Rich Text format and when you use Microsoft Word as your e-mail editor. Microsoft acknowledges this issue in KB article 278061.


Microsoft’s recommended fix for this is to use the Plain Text option when sending to users who experience this problem. You can change options for a contact so that any mail destined for that user is always sent in plain text, however that will have no effect on replies/forwards. You can adjust this on a per-message basis in Outlook 2003 by selecting “Plain Text” from the format drop down list on the toolbar in a new or reply/forward message (the default is usually HTML). In Outlook 2007, you can change this by selecting the Options tab from the Tool Ribbon, and clicking the Plain Text button. Note that HTML formatted messages generally will NOT cause this behavior, except in cases where users have configured Outlook to use message backgrounds and/or graphics (such as in their signature block).

While the above workarounds may resolve this issue, we have seen instances where messages explictly configured to use either the Plain Text or HTML formats continue to result in recipients receiving winmail.dat attachments. We believe this may be related to the format of messages that you receive from non-Central Exchange users - specifically that Outlook may be "remembering" what format you received a message in, and may be attempting to use that format for subsequent e-mails to that original sender. In such cases, you may be able to achieve some resolution by opening a new message, typing the first few characters of a recipients name in the To field (enough that Outlook begins to suggest the recipient's name to you), arrowing down to that entry in the suggestion list, and pressing the Delete key to remove that suggestion from the list. Doing this will remove this recipient from Outlook's "nickname cache", and will remove any settings Outlook has remembered to use when sending to this user.

We recognize that this can be an inconvenience for those users who have migrated to Central Exchange. The next version of Exchange/Outlook, due to be released later in 2009, may bring about a resolution to this issue. In the meantime, we have had success dealing with individuals experiencing this issue on a case-by-case basis. If you or your colleagues encounter a CSU user who is sending attachments such as this, please have them get in touch with us at, and we'll be happy to troubleshoot and resolve their specific situation.

Last updated March 6, 2009