Winmail.dat is actually an Outlook feature that preserves the formatting of the mail body(esp RTF) and as you rightly mentioned that this is from the senders end, hence unfortunately we cannot do much from our end.
Though as a workaround if you can’t ask the senders to send the mails in proper format, you can make your Outlook receive all mails in Plain Text format by following the below mentioned steps:
- Start Outlook.
- Click the File tab in the Ribbon, and then click Options on the menu.
- Click Trust Center on the Options menu.
- Click the Trust Center Settings tab.
- Click E-mail Security.
- Under Read as Plain Text, click to select the Read all standard mail in plain text check box.
- To include messages that are signed with a digital signature, click to select the Read all digitally signed mail in plain text check box.
Please revert if any clarifications required.