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.
Scenario 1: Change the message format for external contactsTo change the message format for an external contact that was added to Exchange Online, follow these steps:
- Connect to Exchange Online by using remote PowerShell. For more info about how to do this, go to the following Microsoft website:
- Run the following Windows PowerShell commands to configure the message format as Text Only:
Set-MailContact <ExternalEmailAddress or GUID> -UseMapiRichTextFormat Never
Set-MailContact -Identity <ExternalEmailAddress or GUID> -UsePreferMessageFormat $True
- 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 domainThis 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.
- Connect to Exchange Online by using remote PowerShell. For more information about how to do this, go to the following Microsoft website:
- Run the following Windows PowerShell command to create a remote domain for an external domain:
New-RemoteDomain -Name <Name of External Domain> -DomainName
- Run the following Windows PowerShell command to prevent messages from being sent in rich text format:
Set-RemoteDomain -Identity <Name of Domain> -TNEFEnabled $false
- Run the following WindowsPowerShell command to check that the setting was applied:
Get-RemoteDomain -Identity <Name of Domain>| Select TNEFEnabled