If you use Mac Mail as your primary server host, you may need to generate an app-specific password to connect with ARTERNAL. Below is a guide to getting that password.
How to generate an app-specific password
Navigate to appleid.apple.com from your web browser and sign in with your Apple ID and Password.
(We would recommend using Chrome browser as Safari browser has had issues with generating app-specific passwords.)
Verify your identity with two-factor authentication.
After logging in, select App-Specific Passwords
Click on Generate an app-specific password
Enter a label for the password
Screenshot and copy and paste that password
Then return to ARTERNAL smart mail settings and click on Link Account to non-Google email providers
In the pop-up window, select iCloud
Copy and paste your app-specific password to log in.