Thunderbird can't login to server

Hello filippo166,

Thank you for taking the time to post your question to Apple Support Communities. We understand that you're receiving an error when attempting to set up your iCloud email address in a third-party email client.

You mentioned the credentials and names in the email configuration being taken from your @icloud.com address, which is correct. To confirm, does the incoming mail server "Username" reflect only the name part of the iCloud email address? As mentioned in Mail server settings for iCloud email clients the user name for the incoming mail service should be "the name part of your iCloud email address (for example, emilyparker, not emilyparker@icloud.com)." The outgoing mail server should reflect the full iCloud email address.

If you've already confirmed each of these settings are correct, let us know if you're able to successfully access your iCloud email here: Sign in to iCloud or whether you're able to set up the iCloud email using a different third-party email client.

If you're able to use your iCloud email with other clients, as well as the iCloud website, reaching out to the developer of the software you're using for additional support is recommended. Check out their support site here: Thunderbird Support

Cheers!