How do I prevent my Outlook from sending winmail.dat files?
There are two possible things that cause your Outlook to send winmail.dat files.
The first is that your settings are set to send emails in Rich Text Format, which you can change with the following steps:
Click File in Outlook.
Go to the Mail tab.
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.
The second cause is because your saved contacts have their own specific settings which will override your Outlook settings. You will need to delete your saved contact if you're experiencing this issue with them.
If you're experiencing this issue with specific recipients who aren't a saved contact, you can follow these steps to manually alter the sending preferences:
Open an email from the desired recipient or compose a new email to them.
Click on the email address with the right mouse button.
Select Outlook Properties from the menu that appears.
Make sure that "Let Outlook decide the best sending format" or "Send Plain Text" is selected under Internet format