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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?

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

Fix winmail.dat attachment in Office 365 login

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.

Fix winmail.dat attachment in Office 365 EAC

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.

Fix winmail.dat attachment in Office 365 edit settings

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.

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

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 »

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

Keep reading: Outlook keeps asking for password after migration »


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!

Did you enjoy this article? You may also like Delete folder in Exchange Online from all mailboxes. Don’t forget to follow us and share this article.



ALI TAJRAN is a passionate IT Architect, IT Consultant, and Microsoft Certified Trainer. He started Information Technology at a very young age, and his goal is to teach and inspire others. Read more »

This Post Has One Comment

  1. Exchange admin center console has a new UI or old UI option (2020-12-28). This setting can be changed from new UI, the setting can be saved, but, after save (with or without refresh) the UI shows that setting is still ‘follow user settings’. In the old UI, the admin console shows that the change is made and can be changed. So make sure that you are using old UI, if you are unable to change this setting.

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