Winmail.dat Reader

Exchange Server and Microsoft 365: stop sending winmail.dat to Internet recipients

Even when Outlook is configured correctly, messages that leave your organization through Exchange can still reach external recipients with a winmail.dat attachment, because the server decides how they are sent. An administrator can control this on the server. This page applies to Exchange Server 2013 and later and to Exchange Online (Microsoft 365).

The old procedure for the Exchange 4.0/5.5 Internet Mail Connector is obsolete and not covered here.

How Exchange decides

Exchange chooses the format in this order: first the setting of the remote domain, then, if the domain follows user settings, the setting of the recipient (mail contact or mail user), and finally the Outlook settings of the sender. A remote domain set to Never therefore wins over everything else. This is the default behaviour. If UsePreferMessageFormat is set to $true on a recipient, that recipient's own format settings override the remote domain.

All external recipients

In the Exchange admin center, go to Mail flow > Remote domains, edit the Default domain and set Use rich-text format to Never. The same with PowerShell:

Set-RemoteDomain Default -TNEFEnabled $false

One external domain

Create a remote domain for it and turn TNEF off for that domain only:

New-RemoteDomain -Name "Partner" -DomainName partner.example.com
Set-RemoteDomain Partner -TNEFEnabled $false

One external recipient

Set the rich-text option of the mail contact (or mail user) to Never:

Set-MailContact "Jane Doe" -UseMapiRichTextFormat Never

Check the result

Confirm that the setting was applied. Changes may take a few minutes to take effect and do not change messages that were already sent.

Get-RemoteDomain Default | Format-List Name,TNEFEnabled
Get-MailContact "Jane Doe" | Format-List UseMapiRichTextFormat

Based on the Microsoft articles: How to specify the email message format that's used for external recipients, TNEF conversion options