First, make sure you can properly log into https://outlook.office365.com/. Usually, that means you must have an Office 365 Business or Office 365 Enterprise account in order to use Zapier. You won't be able to connect to a personal or education account.
These accounts do not work:
If you're getting and error that says
The user or administrator has not consented to use the application with ID 'XXXXX', this means that your Microsoft account doesn't have the right permissions to connect to Zapier. You'll need to have your Office 365 administrator grant you access to connect to third parties.
Microsoft automatically expires their refresh token (what we use to connect to them) if any of the following occur:
Unfortunately, there is no way around these limitations. When your token expires, you will need to reconnect your account to Zapier. You can do that by going to the Connected Accounts page and clicking the reconnect button next to Office365, then typing in your credentials.