To stop Outlook from sending winmail.dat attachments, set your new emails to compose in HTML or Plain Text format. Also, when sending Rich Text emails to internet recipients, make sure they are converted to HTML format or Plain Text format.
If you aren’t sure how to do that, follow our instructions below.
Why does Outlook send winmail.dat attachments?
When composing an email in Outlook, there are 3 ways to format your email: Plain Text, Rich Text, or HTML. By default, Outlook uses HTML format when composing new emails.
The winmail.dat attachment is added to Outlook when you send an email using the Rich Text format that has text enhancements such as bolding, italics, font sizes or any other features that are available with Rich Text format. The file is attached to the email because it contains instructions on how other email clients should format the text.
This only happens when composing in Rich Text format because Plain Text format doesn’t have the ability to add text enhancements, and HTML format includes all the enhancements in the HTML code itself, so there is no need for a winmail.dat file.
The winmail.dat problem happens with all versions of Outlook from 2003 onwards.
How to Prevent Winmail.Dat Attachments from Being Sent in Outlook
Open Outlook, click on File > Options.
Click on Mail from the left menu.
Under the Compose messages section, change the “Compose messages in this format” dropdown box to HTML or Plain Text.
Scroll down until you see the Message format section.
In the “When sending messages in Rich Text format to Internet recipients”, ensure you select either Convert to HTML format, or Convert to Plain Text format.
Click OK to close the Options window.
Preventing Winmail.Dat Attachments in Outlook 2003 – 2007
Open Outlook, click on Tools > Options.
Click on the Mail Format tab at the top.
Under Message format, change the “Compose in this message format” dropdown box to HTML or Plain Text.
Click OK to close the Options window.
Using the Windows Registry to Stop Winmail.Dat Attachments from Being Sent
Warning: Editing your Windows registry incorrectly or making a mistake can cause irreversible damage to your operating system. It’s not advisable to edit your registry manually unless you absolutely have to. Only use this method if the above instructions don’t work and you’re comfortable editing the registry.
Ensure all applications on your computer are closed, including Outlook.
Right click on the Windows logo in the bottom left corner, click Run.
Alternatively, press the Windows key + R to open the Run box.
In the text box, type “regedit” and press Enter, or click OK.
Navigate to the following path:
HKEY_CURRENT_USER\Software\Microsoft\Office\<office version number>\Outlook\Preferences
The <office version number> corresponds to the following versions of Microsoft Office:
Outlook 2019/2016 – 16.0
Outlook 2013 – 15.0
Outlook 2010 – 14.0
Outlook 2007 – 12.0
Right click on Preferences, then select New > DWORD (32-bit) Value.
The name for the new registry key should be “DisableTNEF”. Once you have added the name, hit Enter to apply the changes.
Right click on the new DisableTNEF registry key and click Modify.
Set the Base to Hexadecimal.
Set the Value data to 1.
Close the Registry Editor window, and you’re done! Outlook should no longer be sending winmail.dat attachments.
If you’re still experiencing winmail.dat problems…
If you’ve completed the above steps and you’re still experiencing winmail.dat problems, you’ll need to remove the autocomplete entry for all of your recipients. Autocomplete is when you start typing an email address in a new message, and their email address comes up as a suggestion.
The sending format is saved for every contact in your autocomplete list. So, even after you’ve changed the sending preferences using the above instructions, some recipients that you’ve previously sent emails to may still get the winmail.dat attachment if you dont clear the autocomplete list.