An Office 365 admin wants to specify the email message format that's used for external recipients to prevent Winmail.dat attachments

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INTRODUCTION

This article describes how Microsoft Office 365 admins can change the message format so that messages that are sent from Office 365 users to external recipients don’t contain the Winmail.dat attachment.

By default, email messages that are sent from Exchange Online in Office 365 use the Transport Neutral Encapsulation Format (TNEF) format. Messaging systems that aren't based on Microsoft Exchange may be unable to interpret messages that use this rich text format. If the recipient’s messaging system can’t process this format, a file attachment that's called Winmail.dat is added to the message.

Office 365 admins can use Windows PowerShell to change the message format to prevent the Winmail.dat attachment from being sent to external recipients.

PROCEDURE

To change the message format to prevent Winmail.dat attachments, use one of the following methods.

Scenario 1: Change the message format for external contacts

To change the message format for an external contact that was added to Exchange Online, follow these steps:
  1. Connect to Exchange Online by using remote PowerShell. For more info about how to do this, go to the following Microsoft website:
    Connect to Exchange Online Using Remote PowerShell
  2. Run the following Windows PowerShell command to configure the Message Format to Text Only:
    Set-MailContact <ExternalEmailAddress or GUID> -UseMapiRichTextFormat Never
  3. Run the following Windows PowerShell command to confirm that the message format was applied:
    Get-MailContact | Select <ExternalEmailAddress or GUID> | Select UseMapiRichTextFormat

Scenario 2: Change the message format for all messages that are sent to a specific domain

This method requires you to create a remote domain object in Exchange Online to control how messages are sent to external domains. You can also use this method to change the message format for messages that are sent to coexistence domains.
  1. Connect to Exchange Online by using remote PowerShell. For more information about how to do this, go to the following Microsoft website:
    Connect to Exchange Online Using Remote PowerShell
  2. Run the following Windows PowerShell command to create a remote domain for an external domain:
    New-RemoteDomain -Name <Name of External Domain> -DomainName
  3. Run the following Windows PowerShell command to prevent messages from being sent in rich text format:
    Set-RemoteDomain -Identity <Name of Domain> -TNEFEnabled $false
  4. Run the following WindowsPowerShell command to check that the setting was applied:
    Get-RemoteDomain -Identity <Name of Domain>| Select TNEFEnabled

MORE INFORMATION

For more info about specific Windows PowerShell cmdlets, go to the following Microsoft websites:
Still need help? Go to the Office 365 Community website.

Properties

Article ID: 2487954 - Last Review: October 4, 2014 - Revision: 25.0
Applies to
  • Microsoft Exchange Online
Keywords: 
o365 o365a o365p o365e o365m o365022013 KB2487954

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