What are Winmail.dat files?
Are recipients of your emails complaining about a mysterious attachment called “winmail.dat” (of the even more mysterious content type “application/ms-tnef”), which they cannot open, no matter what they try?
It is not your fault. It is your Outlook’s fault, in a way.
If Outlook sends a message using the RTF (Rich Text Format) format (which is practically not used outside Outlook and Exchange) for bold text and other text enhancements, it includes the formatting commands in the winmail.dat file. Receiving email clients that do not understand the code therein display it as a stale attachment. To make matters worse, Outlook will also commonly pack other, regular file attachments in the winmail.dat file.
Fortunately, you can get rid of winmail.dat issue altogether by making sure Outlook does not even try to send mail using RTF (Rich Text Format).
Outlook 2010, 2013, 2016 & Office 365 – Change Winmail.dat attachments from being sent
Select Options > Mail
Under the heading Compose messages, make sure Compose messages in this format has HTML or Plain Text selected
Scroll down and under the heading Message format, make sure When sending messages in Rich Text format to Internet recipients is set to Convert to HTML format or Convert to Plain Text format
Outlook 2003 & 2007 – Change Winmail.dat attachments from being sent
Select Tools > Options and select the Mail Format tab
Under Compose in this message format:, make sure either HTML or Plain Text is selected
Select Internet Format and 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 and again, click OK
NOTE: 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.