The latest Outlook (Microsoft 365 for Win and Mac, Outlook on the web) update has added text predictions so when you are composing a message, it will suggest words or phrases to come next.  You can choose to ignore the suggestions or press tab or right arrow key to accept the suggestions.  For more details, see Editor text predictions in Outlook 

If you don't like the feature, you can turn it off:

Windows 

1. While composing a new message, choose File | Options
2. Uncheck the "Show text predictions while typing" check box. 

Mac

1. Select Preferences | AutoCorrect | Text Completion.
2. Uncheck the "Show text predictions while typing" check box.

Outlook on the web

1. Select Settings | View all Outlook settings | Mail | Compose and reply.
2. Under Text predictions, uncheck the "Suggest words or phrases as I type" check box.

Outlook on the web (Chrome and Microsoft Edge) also has suggestions for grammar, spelling, text refinement -- see Check grammar, spelling, and more in Outlook.com and Outlook on the web for more information