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Knowledge Base

Web Mail And MS-TNEF

Users who do not use Microsoft Outlook may receive file attachments with a filename of winmail.dat or application/ms-tnef. MS-TNEF (Microsoft Transportation Neutral Encapsulation Format) is a proprietary Microsoft MIME format that is generated by senders who use Outlook, and have one or more of the following Outlook settings in place:

1. Tools > Options > Mail Format > Compose in this message format: - Should be: HTML or Plain Text
2. Tools > Options > Mail Format> Use Microsoft Word to edit e-mail messages - Should be: Unchecked ( )
3. Always Send To This Recipient In Microsoft Exchange Rich-Text Format - Should be: Unchecked (for each Address Book contact)

If you are unable to open an MS-TNEF attachment, please ask the sender to adjust their Outlook settings, and resend the message. (Mac OS users can also try using a free utility, TNEF's Enough, which attempts to decode MS-TNEF files.) Non-Microsoft email programs such as Web Mail and Thunderbird do not decode MS-TNEF as the format is exclusive to Microsoft.

This Knowledge Base article references Microsoft Knowledge Base Article - 138053, How to Prevent the Winmail.dat File from Being Sent to Internet Users. cannot control how users send email in the event of a winmail.dat issue. We suggest you contact the sender and ask they adjust their settings to prevent this from happening.