Exchange - External Users receiving Winmail.dat

Issue: External users receive an attachment named "Winmail.dat"

Cause: There is a transport rule applied to the tenant which is wrapping the email in a attachment.

Explanation: There are 3 actions (Wrap, Insert and Reject) when an email disclaimer is added while sending the email outside the organization. 

Wrap: The original message is wrapped in a new message envelope, and the disclaimer text is inserted into the new message. This is the default value.
• Subsequent mail flow rules are applied to the new message envelope, not to the original message. Therefore, configure these rules with a lower priority than other rules.
• If the original message can't be wrapped in a new message envelope, the original message isn't delivered. The message is returned to the sender in an NDR.

Ignore: The rule is ignored and the message is delivered without the disclaimer

Reject: The message is returned to the sender in an NDR.

Solution: Leverage either of the below steps in Exchange Admin Center or Powershell to setup rules in Exchange Online for inserting disclaimers

Use the EAC to add a disclaimer or other email header or footer

1. Open the EAC and go to Mail flow > Rules.

2. Click Add , and then click Apply disclaimers.

   

3. In the New rule window that appears, enter a unique name the rule.

4. In the Apply this rule if box, select the conditions for displaying the disclaimer. For example, select The recipient is located condition, and then select Outside the organization. If you want this rule to apply to every message that enters or leaves your organization, select [Apply to all messages].

5. Next to the Do the following box, select Enter text to enter the text of your disclaimer.

6. Click Select one, and select one of the Fallback options if the disclaimer can't be added.

7. Specify the audit severity level to assign the severity level that appears in the message log.

8. Select the mode for the rule. Select Enforce to turn on the disclaimer immediately, or select Test without Policy Tips to put a message in the message tracking log instead of adding the disclaimer.

9. If you have additional conditions or exceptions that you want to add, select More options at the bottom of the page, which will show additional settings. For example, to add the exception that prevents multiple disclaimers being added in an email conversation, select Add exception and then select The subject or body > Subject or body matches these text patterns, and then specify the words or phrases in your disclaimer. Or, to put your disclaimer at the top of the email message instead of the bottom, in Do the following, select Apply a disclaimer to the message > prepend a disclaimer.

10. When you're finished, click Save.

Use Exchange Online PowerShell to add a disclaimer or other email header or footer

Use the New-TransportRule cmdlet to create the disclaimer rule. For detailed parameter information, see Mail flow rule conditions and exceptions (predicates) in Exchange Online.

This example creates a new mail flow rule that adds a disclaimer with an image to the end of all email messages that are sent outside the organization.

New-TransportRule -Name "External Disclaimer" -SentToScope NotInOrganization -ApplyHtmlDisclaimerText "<h3>Disclaimer Title</h3><p>This is the disclaimer text.</p><img alt='Contoso logo' src='http://www.contoso.com/images/logo.gif'>"

This example creates a new mail flow rule that adds an advertisement for one month to the beginning of all outgoing messages.

New-TransportRule -Name "March Special" -Enabled $true -SentToScope NotInOrganization -ApplyHtmlDisclaimerLoc

Microsoft Documentation: Organization-wide message disclaimers, signatures, footers, or headers in Exchange Online | Microsoft Docs

Case: 17999