''re :Comcast added email settings security with a box to check to allow third party to access.'' Have you selected that checkbox to allow third party access ? If no, then select it - at least logon to the webmail account via a browser and check it is selected otherwise Xfinity will block you. Have you reset a new password in comcast/Xfinity webmail account ? If yes, have you accessed the stored password in Thunderbird and updated it for both the imap and the smtp ? If no, then get new password saved: * Settings > Privacy & Security * Password section * click on 'Saved Passwords' * Click on 'Show Passwords' * For both incoming imap line and outgoing smtp line - * right clickon line and select 'Edit Password' When completed - restart Thunderbird. If still a problem then post an image of the error message.