How to prevent internal emails from Microsoft Exchange generating the 'winmail.dat' attachment

Technical Articles ID: KB96445
Last Modified: 2023-07-11 20:57:46 Etc/GMT

Environment

Security for Microsoft Exchange 8.x

Windows Server 2022, 2019, 2016, 2013
Exchange Sever 2019, 2016, 2013

Summary

This article describes how to prevent Microsoft Exchange from renaming and clubbing emails with the winmail.dat attachment.

This process is required if you created custom file filtering rules to block *.dat attachment types.

Problem

When an Exchange user sends mail to any recipients, the attachment is converted to winmail.dat. This issue occurs with Outlook versions 2003 and later.

Cause

The Transport Neutral Encapsulation Format (TNEF) is a Microsoft-specific format for encapsulating MAPI message properties. A TNEF message contains a plain-text version of the message and an attachment that packages the original formatted version of the message.

Typically, this attachment is named winmail.dat. A MAPI-compliant email client that fully understands TNEF, such as Outlook, processes the winmail.dat attachment and displays the original message content without ever displaying the winmail.dat attachment. An email client that doesn't understand TNEF might present a TNEF message like the plain-text version of the message.

The message contains an attachment named winmail.dat, win.dat, or some other generic name such as att_nnnnn_.dat or att_nnnnn_.eml, where 'nnnnn' represents a random number.

Solution

A TNEF-encoded message contains both 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 includes 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

Follow these steps to control TNEF in messages: Microsoft Support.

For additional information about how to prevent internal emails from Microsoft Exchange from generating attachment names such as winmail.dat, see the following:

Affected Products

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How to prevent internal emails from Microsoft Exchange generating the 'winmail.dat' attachment

Environment

Summary

Problem

Cause

Solution

Affected Products

Languages:

Title

Title

Knowledge Center

How to prevent internal emails from Microsoft Exchange generating the 'winmail.dat' attachment

Environment

Summary

Problem

Cause

Solution

Affected Products

Languages:

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Title

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