If using MAC Mail Client an app password is required. Follow the steps in section 4.6. Once section 4.6 has been completed you will have generated an app password. Use this app password for you MAC Mail Client.
1. Go to 'System Preferences -> Internet Accounts - > Exchange'.
2. Enter your Username, Email Address and enter the App Password as your password.
3. Select the option you wish to enable and click 'Done'.
4. If you have iCloud enabled for your KeyChains, these settings and app password will apply to the same application across all your additional devices. On additional devices you will see the following pop-up. Click 'Turn On' to enable MAC mail client on additional devices.If iCloud is not enabled you will need to setup an app password for every application.