Fix Winmail.dat Attachment Issue in Microsoft 365: Complete Administrator Guide
When external recipients receive Winmail.dat instead of your PDF or document attachments, the culprit is Microsoft's TNEF encoding. This guide walks IT administrators through multiple solutions to permanently fix this issue in Microsoft 365, from Exchange Admin Center configuration to PowerShell commands and Outlook client settings.
The Problem
The winmail.dat attachment issue is a long-standing problem in Microsoft email environments, especially when using Microsoft 365 and Outlook.
Instead of receiving normal attachments such as PDF or DOCX files, recipients sometimes see a mysterious file named winmail.dat, which they cannot open.
This issue is more than an inconvenience: it disrupts business communication, affects external recipients using Gmail, Apple Mail, or mobile devices, and generates unnecessary support tickets for IT teams.
Step-by-Step Guide
Access Exchange Admin Center
Disable TNEF encoding for all external domains at the Exchange Online organization level, forcing automatic conversion of Rich Text messages to HTML before delivery.
Navigate to the Microsoft 365 admin center at admin.microsoft.com. From the left navigation panel, expand Admin centers and select Exchange to open the Exchange Admin Center. Alternatively, access it directly at admin.exchange.microsoft.com.
Navigate to Remote Domains
Use Exchange Online PowerShell to disable TNEF encoding and optionally configure character sets for proper international character handling.
In the Exchange Admin Center, expand Mail flow in the left navigation panel and select Remote domains. This section controls how Exchange Online handles outbound email to external domains.
Edit the Default Remote Domain
Identify and reconfigure Mail Contacts in Exchange Online that have TNEF enabled for their individual addresses.
Locate the remote domain named Default with an asterisk (*) as the domain name. This wildcard entry applies settings to all external domains not explicitly configured with their own rules. Click on the Default entry to open its settings.
Modify Text and Character Set Settings
Scroll down to the Text and character set section. Locate the Rich-text format setting. By default, this may be set to Follow user settings or Use Exchange rich-text format. Change this setting to Never use to force Exchange Online to convert all Rich Text messages to HTML before sending them externally.
Save Changes
Click Save to apply the new configuration. The change takes effect within a few minutes, though Microsoft documentation indicates it may take up to 15 minutes to propagate fully across the service.
How It Works
Microsoft Outlook and Exchange use a proprietary encoding method called Transport Neutral Encapsulation Format (TNEF) when sending emails formatted as Rich Text Format (RTF). TNEF packages the email's formatting, embedded objects, and attachments into a special binary file. When the recipient's email client understands TNEF (another Outlook or Exchange user), everything displays correctly. However, when the recipient uses Gmail, Apple Mail, Yahoo, or any non-Microsoft email client, their system cannot decode the TNEF package and displays it as an attachment named Winmail.dat or occasionally Win.dat.
The Winmail.dat file actually contains the original attachments and rich formatting, but without specialized tools, recipients cannot access this content. From the sender's perspective, everything appears normal because Outlook shows the email as successfully sent with proper attachments. The sender often remains unaware of the problem until the recipient reports they received an unusable file instead of the expected document.
The Winmail.dat attachment issue stems from Microsoft's proprietary TNEF encoding encountering email systems that cannot interpret it. By configuring Exchange Online Remote Domains to disable TNEF (either through the Admin Center or PowerShell), administrators can permanently resolve this issue for their entire organization. For specific contacts or edge cases, Mail Contact configuration and Outlook client settings provide additional control.
Conclusion
The winmail.dat issue is a classic example of legacy email behavior conflicting with modern communication standards.
By disabling TNEF in Microsoft 365, organizations can eliminate this problem permanently, reduce user frustration, and improve email interoperability across platforms.
For IT administrators, this fix is simple, safe, and highly recommended.
Frequently Asked Questions
Winmail.dat is caused by Outlook sending emails using Microsoft’s proprietary TNEF format instead of standard MIME.
No. It is not malicious, but it can hide legitimate attachments from recipients.
Yes. Disabling TNEF is considered best practice for external email compatibility.
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