If multi-factor authentication is configured for your account, access to your email on mobile devices or in Outlook will be controlled by App Passwords. If a device or Outlook requests the password, you can generate a new app password with this process.
1. Login to your Office365 account at http://login.microsoftonline.com
2. Click on the cog wheel icon at the top-right of the page and select 'Office 365 Settings'.
3. Go to 'Settings'. Under 'Additional Security Verification' at the bottom of the page, click the link to 'Update your phone numbers used for account security'.
4. Your phone may receive an SMS with a verification code depending on when you last signed in.
5. It's not obvious, but there are two sections here:
a. Additional Security Verification
b. App Passwords
6. Go to 'App Passwords'.
7. Click 'Create' to create a new one.
8. Write a description of the App that is accessing your email account, e.g. 'Outlook', and a password will be generated.
9. If it is for Outlook, you can just copy the password and paste it in the Outlook window that is requesting the password.