Fix Winmail.dat Attachment Issue – Exchange Online/Office 365 2026

This method disables TNEF format through the Exchange Admin Center interface.

1. Sign in to Microsoft 365 Admin Center with administrator credentials
2. Navigate to Admin centers → Exchange
3. In Exchange Admin Center, expand Mail flow in the left navigation
4. Click on Remote domains
5. Select Default from the remote domains list
6. Click the Edit (pencil) icon
7. Navigate to the Message format tab
8. Under Use rich-text format, select Never
9. Change Character set to Unicode (UTF-8)
10. Change Non-MIME character set to Unicode (UTF-8)
11. Click Save to apply changes

Configure remote domain settings using PowerShell for more precise control.

1. Install the Exchange Online PowerShell module if not already installed:

   Install-Module -Name ExchangeOnlineManagement -Force

2. Connect to Exchange Online PowerShell:

   Connect-ExchangeOnline -UserPrincipalName admin@yourdomain.com

3. List current remote domains to verify configuration:

   Get-RemoteDomain | Format-Table -AutoSize Name, DomainName, TNEFEnabled

4. Check the current TNEF settings for the default domain:

   Get-RemoteDomain -Identity "Default" | Select-Object Id, TNEFEnabled, CharacterSet, NonMimeCharacterSet

5. Disable TNEF and set UTF-8 character encoding:

   Set-RemoteDomain -Identity "Default" -TNEFEnabled $false -CharacterSet utf-8 -NonMimeCharacterSet utf-8

6. Verify the changes were applied:

   Get-RemoteDomain -Identity "Default" | Select-Object Id, TNEFEnabled, CharacterSet, NonMimeCharacterSet

7. Disconnect from Exchange Online:

   Disconnect-ExchangeOnline

Adjust individual Outlook client settings to prevent rich text format for internet recipients.

1. Open Outlook desktop application
2. Navigate to File → Options
3. Select Mail from the left sidebar
4. Click Editor Options button
5. In the Editor Options dialog, click Advanced
6. Scroll down to International options
7. Uncheck Automatically select encoding for outgoing messages
8. Click OK to close Editor Options
9. Back in Mail options, scroll to Message format section
10. Set Compose messages in this format to HTML
11. Click Internet Format button
12. Select Convert to HTML format
13. Set Encoding to Unicode (UTF-8)
14. Click OK and then OK again to save settings

Create targeted remote domain configurations for specific domains experiencing Winmail.dat issues.

1. Connect to Exchange Online PowerShell:

   Connect-ExchangeOnline

2. Identify the problematic recipient domain (e.g., gmail.com, yahoo.com)
3. Create a new remote domain for the specific domain:

   New-RemoteDomain -Name "Gmail Domain" -DomainName "gmail.com"

4. Configure the new remote domain to disable TNEF:

   Set-RemoteDomain -Identity "Gmail Domain" -TNEFEnabled $false -CharacterSet utf-8 -NonMimeCharacterSet utf-8

5. Set additional formatting options:

   Set-RemoteDomain -Identity "Gmail Domain" -ContentType MimeHtmlText -LineWrapSize Unlimited

6. Verify the new remote domain configuration:

   Get-RemoteDomain -Identity "Gmail Domain" | Format-List

7. Repeat steps 3-6 for other problematic domains as needed

Create transport rules to force plain text or HTML format for external emails with attachments.

1. In Exchange Admin Center, navigate to Mail flow → Rules
2. Click Add a rule → Create a new rule
3. Name the rule: Prevent Winmail.dat for External Recipients
4. Set condition: The recipient is located → Outside the organization
5. Add condition: The message → has an attachment
6. Set action: Modify the message properties → Set the message format to HTML
7. Add action: Set message header with:
   • Header name: Content-Type
   • Header value: text/html; charset=utf-8
8. Set rule priority to 0 (highest priority)
9. Enable the rule and click Save
10. Alternative PowerShell approach:

    New-TransportRule -Name "Block TNEF External" -SentToScope NotInOrganization -HasAttachment $true -SetHeaderName "X-MS-Exchange-Organization-DisableTnef" -SetHeaderValue "true"