Some email providers require an app password in order to connect mailivery to it.
Below you can see an generic example of how to generate an app password with Yahoo mail. The way an app password gets generated is always similar.
Yahoo app password
1. Sign in and go to your Account security page. To do that you have to click on "account info" while hovering on your account icon.
2. Go to "Account Security".
3. Click on "Manage app passwords"
4. Select the app "other app" and enter the custom name "mailivery"
5. Click generate and copy the app password.