An app-specific password (some providers call it an app password) is a separate password your email provider creates for one app, such as Newton. iCloud, Yahoo, AOL and Fastmail require one: Newton can't sign in to them with your normal password.
Steps
Open your provider's instructions from the list below.
Follow them to create a new app password. If you're asked for a name, use one you'll recognize, like "Newton".
Copy the password.
In Newton, paste it into the password field when you add the account, instead of your normal password.
Setting up Newton on a phone for the first time? The iCloud and Yahoo screens have an Open iCloud Mail account settings or Open Yahoo Mail account settings link that takes you to the right account page.
Good to know
Apple only lets you create app-specific passwords when two-factor authentication is on for your Apple Account.
Zoho may need an application-specific password if you use two-factor authentication with Zoho. See Zoho's instructions.
If you delete the app password at your provider, Newton can't connect to that account anymore. Create a new one and reconnect the account.
Newton stores the password encrypted on its servers, so your other devices can connect without asking for it again.