Do you ever see strange attachments to your emails named winmail.dat? This is a relic from the days when Microsoft monopolized the desktop operating system and decided to create it’s own format for the transmission of email (completely ignoring already well established conventions). The called this oddity Transport Neutral Encapsulation Format (or TNEF for short).
Some email readers (like OSx’s Mail or Mozilla’s Thunderbird) and email transfer agents (the server software that passes your email along until it reaches your mailbox) don’t support TNEF and sometimes strange things happen which cause emails to display with winmail.dat attachments.
The best way to avoid such problems is to disable Rich Text Formatting in Exchange Online completely. This is done by logging in with an administrative login, navigating to ‘mail flow’, selecting the remote domains tab, double clicking on the ‘Default’ domain and then selecting ‘Use rich-text format: Never’.