How to open a file in winmail.dat format in Outlook?

You have received a file in Outlook or Thunderbird as an attachment winmail.dat format ? Generally it is possible to have emails with attached files in .dat format.

You may not know it, but .dat files can be opened with some software. We explain everything in this tutorial to prevent your recipients from receiving emails in Winmail.dat format.

In fact, you are thinking of sending a file in .ppt or even .docx or .xlsx format, but ultimately a file in winmail.dat format arrives.

Why in Outlook do I have Winmail.dat files?

When you send an email with the Microsoft Outlook client , a file called winmail.dat could be attached, as an attachment This file contains formatting data in Microsoft proprietary format TNEF0 File in .dat format is not recognized by other email clients (such as Outlook or Thunderbrid).

This type of attachment in the format winmail.dat is usually unreadable in Outlook. Sometimes when connecting to the webmail the attachment is quite readable.

From my tests I noticed that the tools that allow you to send emails directly via Outlook have concerns.

outlook winmail.dat
Here I received an email in Winmail.dat format

Solve the Winmail.dat problem

Obviously this problem does not only affect Outlook, Thunderbird users may be affected.

If your recipients notify you of an attachment problem, your emails with attachments arrive in winmail.dat then you can try the following manipulation.

In Outlook change the format of the raw file in the options:

outlook options

Then we will modify the format of the composition of the messages:

plain text outlook

Your attachments should no longer be sent in winmail.dat format

If you use Thunderbird there is an extension

Download the LookOut plugin for Thunderbird

This one goes automatically decode the winmail.dat file in your emails.

Finally for those who use a MAC there is an application TNEF

Once installed on your MAC you must drag the Winmail.dat file into the application.

For those who are on Office vcheck that your Outlook client is up to date (do a search on Windows Update or search for updates within Office)


Prevent Outlook from sending attachments in winmail.dat format

Outlook Heavy Client

Depending on the version of your Outlook client, you must modify the value of the following registry key:

To access the registry, press Windows key + R then enter Regedit

open regedit
enter regedit to open the registry

Then you have to go here

HKEY_CURRENT_USER Software Microsoft Office 16.0 Outlook Preferences

Add a new value key of type DWORD which will be named DisableTNEF.

Then it is necessary enter the value 1

You have to adapt the path of the registry key according to the version of your Office:

  • For office 2010 it is 10.0
  • For the 2013 version the value is 15.0

Office 365 client

To modify the values ​​directly on your portal office 365 Powershell commands must be placed

$UserCredential = Get-Credential
$Session = New-PSSession -ConfigurationName Microsoft.Exchange -ConnectionUri
 -Credential $UserCredential -Authentication Basic -AllowRedirection
Import-PSSession $Session
Set-RemoteDomain Default -TNEFEnabled $false
Remove-PSSession $Session

You will need to identify yourself with a Microsoft “tenan” administrator account.

For those who like the interface it is also possible:

Then in Remote mail flow and domain.

We open the default domain and we will modify the value at the bottom “Use RTF format” for ever”

Open files in .dat format

Files in .dat format can be opened using Notepad or the Notepad ++ application.

The notepad being installed as standard, you can open the file directly with this tool:

outlook winmail.dat open
We will choose the default notepad program
dat notepad
Choose Notepad

Well done your WinMail.dat file can open

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