I've had long conversations with Apple regarding the setup of iCloud email accounts on Thunderbird. The process must include the generation of an App Specific Password using Apples web page : https://appleid.apple.com/account/manage This generates a password in the form aaaa-bbbb-cccc-dddd which is a different password to be used only with one Windows APP. Apple insists this is working with their standard mail server settings preset by all the email client providers. I've been advised to refer to Mozilla Thunderbird and Apple are insistent they can do no more than mail me links to all the instructions I've already read in detail several times. However, I believe the problem lies with Apple but they don't yet know it. I've used separate ASP generated passwords and tried to install an iCloud email on several different email clients: Thunderbird, Mailbird, Outlook and Postbox. All these confirm that the settings seem to be valid but nonetheless the email account fails to load with error messages suggesting the configuration or password (i.e. the ASP) is wrong. The common factor is Apple not Thunderbird. I suggest anyone flummoxed by this awaits the acceptance of this problem by Apple.