What are Winmail.dat files?
Do you have 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?
This is not your fault, it’s Microsoft 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).
How to stop Winmail.dat attachments from being sent in Outlook 2010, 2013, 2016 & Office 365
How to stop Winmail.dat attachments from being sent in Outlook 2003 & 2007
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.