To disable Rich Text Format, and eliminate WINMAIL.DAT transmission, in Exchange Online:
- Log into https://outlook.office365.com
- Click the 3×3 matrix at the upper-left, click Admin
- Scroll down on the left side, open Admin if it’s not opened, click Exchange
- Under the heading Mail Flow, click Remote Domains
- Edit the item Default
- set “Use rich-text format”, to Never.
In Exchange 2010:
- Open the Exchange GUI console,
- Open Organization Configuration, Hub Transport, and the Remote Domains tab.
- Open the Properties of Default/*.
- Browse to the Message Format tab.
- Under “Exchange rich-text format”, choose “Never use”.