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We already talked in this post about the Winmail.dat file, which is part of the proprietary TNEF format used by Exchange and Outlook to encapsulate attachments files, notes and meta-data inside mail messages: the main focus there was figuring out how to open these files when we receive them to other, non-Microsoft e-mail clients. Now we'll approach the same issue from the other side, seeing how we can configure Outlook to stop sending its attachment files using the Winmail.dat.
Why should we do such a thing? The answer is based upon our previous post. Since it's a proprietary format, there's no (native) way to read these attachments for the e-mail receiver, unless he's using Outlook aswell. This cuts out Thunderbird, Eudora, all Android clients, all Mac clients - including Outlook - and more. That's an excellent reason to disable this feature, which is what we're about to do.
We can achieve such result either in a centralized way, using the global MS Exchange settings for external domains, or on client-basis, by changing the options of each Outlook client. The choice, as always, is up to you, depending to your specific network configuration and/or scenario.
Disabling Winmail.dat via Exchange
You can enable or disable TNEF (Transport-Neutral Encapsulation Protocol, which is the format upon which the Winmail.dat file is based as explained here) in the following ways, depending on which Exchange version you're using:
- Exchange Online Dedicated: Open the Exchange Admin Center (EAC) and modify the TNEF conversion options by setting the Mail flow > Remote domains > Edit > Use Exchange rich-text format option to "Never".
- Exchange Online, Exchange 2013 & Exchange 2010: you can disable TNEF either by using the Exchange Management Console (see image below) or via the command-line, by using the TNEFEnabled parameter supported by the Set-RemoteDomain cmdlet to configure the TNEF conversion settings for remote domains. Here's the command to disable TNEF:
Set-RemoteDomain -Identity thedomain -TNEFEnabled $false If you need additional information regarding the Set-RemoteDomain usage, or you want to take a look to all its supported commands, you can read this great KB guide.
To get even more info regarding the TNEF and how to disable/enable it, you can also check out this other KB guide.
Disabling Winmail.dat using Outlook
Select Tools > Options > Email Format > Internet Options: from there, select Convert to HTML format.
Outlook 2010 and 2013
Select File > Options > Mail and scroll down to the bottom. From there, select Convert to HTML format.
If you prefer, you can also disable the TNEF for specific contacts by going into their Outlook Properties: you can do that by double-clicking their e-mail and then selecting "Outlook Properties" from the resulting modal (see screenshot below): from there, you can set the Internet Format options to make Outlook behave as you want to.
IMPORTANT: if you're using this method, you can have some caching issues due. To remove the cache used by Outlook, just begin to type the contact e-mail address until the auto-complete feature kicks in: then, use the arrows to select the contact e-mail and tap DEL to delete it. The cache will be removed as well. Type again the full contact e-mail address and you're set.
Disable TNEF using the Registry Editor
If all the above method fail, you can resort to entirely remove TNEF by altering your registry settings. It will be a rather harsh way of doing that, and we only suggest it to System Administrator who know what they're doing. Take in mind that perform this task will also disable each Outlook TNEF-based feature, such as the built-in Voting System and Meeting Manager.
- Close Outlook
- Launch the Registry Editor from the Start menu by selecting Run, then typing regedit and pressing ENTER.
- navigate through the following registry key:
- Outlook 2007: HKEY_CURRENT_USER\Software\Microsoft\Office\12.0\Outlook\Preferences
- Outlook 2010: HKEY_CURRENT_USER\Software\Microsoft\Office\14.0\Outlook\Preferences
- Outlook 2013: HKEY_CURRENT_USER\Software\Microsoft\Office\15.0\Outlook\Preferences
- Add a new DWORD entry with the following name: DisableTNEF, and give it a value of 1.
- Close the Registry Editor.
- Restart Outlook.
And that's it.