Due to Apple’s new policy of increased security, all iCloud users now need to use App passwords for 3rd party programs like eM Client. Even if you did not enable 2-factor authentication you now need an App password to access eM Client. Here is the official information from Apple support’s newsletter:
Beginning on June 15, app-specific passwords will be required to access your iCloud data using third‐party apps such as Microsoft Outlook, Mozilla Thunderbird, or other mail, contacts, and calendar services not provided by Apple.
If you are already signed in to a third‐party app using your primary Apple ID password, you will be signed out automatically when this change takes effect. You will need to generate an app-specific password and sign in again.
To generate an app-specific password, turn on two-factor authentication for your Apple ID and then follow the instructions below:
• Sign in to your Apple ID account page (https://appleid.apple.com)
• Go to App-Specific Passwords under Security
• Click Generate PasswordFor more information, read Using App-Specific Passwords. If you need additional help, visit Apple Support.
Apple Support