How to stop office 365 sending winmail.dat attachments
Posted by Support Team on 08 February 2016 10:27 PM

If you have used Outlook for any length of time, you’ve probably heard of winmail.dat – you may not have seen it but someone you have sent a message to may have complained about not being able to open an attachment you sent.

In the words of Microsoft:

Do some of your users report that e-mail recipients in external domains can’t open their messages that contain a Winmail.dat attachment? If so, the recipients in the external domain are probably using an e-mail client that doesn’t support the Transport Neutral Encapsulation Format (TNEF). Microsoft Outlook is one of the few e-mail clients that support TNEF-encoded messages, although some third-party utilities can help convert Winmail.dat attachments.


There isn’t an easy way to do this on the web client. Like many things, we have to go down the PowerShell route. So, assuming you haven’t used PowerShell with Office 365, you will need the following:

1) Download and install the Microsoft Online Services Sign-In Assistant for IT Professionals

2) Download and install the Microsoft Online Services Module (step 2)

3) Start up PowerShell on your PC using “Run As Administrator”

You will need to paste in the following commands:

$Session = New-PSSession -ConfigurationName Microsoft.Exchange -ConnectionUri -Credential $LiveCred -Authentication Basic –AllowRedirection

Import-PSSession $Session

Set-RemoteDomain Default -TNEFEnabled $false

The first line asks for your Office 365 (admin account, please!) username and password and connects you to the service.

Line 2 imports the commands you are going to need.

Line 3 prevents TNEF messages being sent outside of your domain.

Setting it to anywhere outside of your domain is a slightly broad brush-stroke, but it does reduce the overhead of having to keep going back and fix it for each domain you discover that cannot cope. But if you really wanted to only disable TNEF for specific domains, you can:

New-RemoteDomain -Name Awkward -DomainName
Set-RemoteDomain Awkward -TNEFEnabled $false

In the first line you define your awkward domain, in the second you prevent TNEF messages going to it.

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