Fix Winmail.dat attachment in Office 365

The recipient is receiving Winmail.dat attachment instead of a PDF attachment when sent from an Office 365 mailbox. How to fix Winmail.dat attachment from showing up, and why is it happening? In this article, you will learn how to prevent Outlook messages from being sent as Winmail.dat in Office 365.

Receiving Winmail.dat instead of attachment

After migrating to Exchange Online in Office 365, the customer reports that recipients see Winmail.dat attachment instead of a PDF file. It’s not happening at every recipient, some do see the attachment as PDF, and some will see the Winmail.dat attachment.

By default, email messages 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 not interpret messages that use this rich text format. If the recipient’s messaging system can’t process this format, a file attachment called Winmail.dat is added to the message.

How to fix Winmail.dat attachment in Office 365?

Sign in to Office 365 Exchange Admin Center with your admin credentials.

Click mail flow in the feature pane and follow with remote domains in the tabs. Click the Default remote domain in the list and click the Edit icon in the toolbar.

Go to the Use rich-text format option and select Never. Change both of the Supported character set to Unicode (UTF-8). Click the button Save when done.

Send an email, and this time it shows as a PDF attachment instead of Winmail.dat. Great job!

Do you want to know how to change the settings with PowerShell instead? Keep reading on how to prevent Winmail.dat attachment in Office 365 with Exchange Online PowerShell.

Fix Winmail.dat attachment with Exchange Online PowerShell

Connect to Exchange Online PowerShell. List the remote domains in the tenant. Only one remote domain is configured, and that’s the name Default.

PS C:\> Get-RemoteDomain | Format-Table -AutoSize

Name    DomainName AllowedOOFType
----    ---------- --------------
Default *          External

The TNEFEnabled value is empty; it means that it’s not configured on Never.

PS C:\> Get-RemoteDomain -Identity "Default" | Select-Object Id, TNEFEnabled, CharacterSet, NonMimeCharacterSet 

Id      TNEFEnabled CharacterSet NonMimeCharacterSet
--      ----------- ------------ -------------------
Default             iso-8859-1   iso-8859-1

Set TNEFEnabled value as false, and adjust both CharacterSet and NonMimeCharacterSet to UTF-8. Verify if it’s configured with the second cmdlet.

Read more on Supported character sets for remote domains in Exchange Online »

PS C:\> Get-RemoteDomain -Identity "Default" | Set-RemoteDomain -TNEFEnabled $false -CharacterSet utf-8 -NonMimeCharacterSet utf-8


PS C:\> Get-RemoteDomain -Identity "Default" | Select-Object Id, TNEFEnabled, CharacterSet, NonMimeCharacterSet 

Id      TNEFEnabled CharacterSet NonMimeCharacterSet
--      ----------- ------------ -------------------
Default       False utf-8        utf-8

I hope that this article helped you to fix the Winmail.dat Outlook issue.

Keep reading: Outlook keeps asking for password after migration »

Conclusion

In this article, you learned how to fix Winmail.dat attachment in Outlook when using Office 365. Check and edit the settings in Office 365 Exchange Admin Center or with Exchange Online PowerShell. After that, verify if it’s working by sending an email. No more Winmail.dat attachments at the recipient inbox!

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