*If after trying all these following steps: Try deleting the autocomplete email address of the user that you are sending to.*
*Ultimately this was the fix for me after trying all of the below fixes*
Fortunately, you can get rid of winmail.dat altogether by making sure Outlook does not even try to send mail using RTF.
Prevent Winmail.dat Attachments from Being Sent in Outlook
To prevent Outlook from attaching winmail.dat when you send an email:
Click File in Outlook.
Go to the Mail category.
Make sure HTML or Plain Text is selected for Compose messages in this format: under Compose messages.
Now make sure Convert to HTML format or Convert to Plain Text format is selected for When sending messages in Rich Text format to Internet recipients: under Message format.
if you use Outlook with an Outlook Mail on the Web (Outlook.com) account, winmail.dat attachments may be sent to people in your address book no matter your Outlook options. This is an issue with Outlook and Outlook Mail on the Web, and you will, alas, need for Microsoft to update the applications to for it to be resolved.
Prevent Winmail.dat Attachments in Outlook 2002-2007
To make sure Outlook 2002 to Outlook 2007 do not attach winmail.dat files:
Step by Step Screenshot Walkthrough
Select Tools | Options… from the menu.
Go to the Mail Format tab.
Under Compose in this message format:, make sure either HTML or Plain Text is selected.
Click Internet Format.
Make sure either Convert to Plain Text format or Convert to HTML format is selected under When sending Outlook Rich Text messages to Internet recipients, use this format:
Click OK again.
Disable Winmail.dat Stubbornly Going to Particular Recipients No Matter the Default
The standard settings for outgoing mail formats in Outlook can be overridden per email address. So, on a per case basis—when somebody complains about an inexplicable “Winmail.dat” attachment after you have made all the right settings changes — you may have to reset the format for individual addresses:
In Outlook 2016:
Make sure the email address is not in your Outlook Contacts.
Outlook 2016 currently offers no way to change sending preferences for email addresses that are assigned to an address book entry.
Open an email from the desired email address or start a new message to it.
Click on the address with the right mouse button.
Select Outlook Properties… from the menu that appears.
In Outlook 2007-2013:
Search for the desired contact in your Outlook Contacts.
Double-click the contact’s email address.
Alternatively, click on the desired email address with the right mouse button and select Open Outlook Properties… or Outlook Properties… from the menu.
Make sure either Let Outlook decide the best sending format or Send Plain Text only is selected under Internet format:.