Comcast IMAP authentication failing despite valid webmail login

@Comcast.net email works in Xfinity webmail with the same username/password. Third Party Access Security is enabled. Account has not migrated to Yahoo. IMAP is configured with imap.comcast.net, SSL, port 993 and the full email address as username but third party mail clients reject the credentials as incorrect. Please check fo an account-level IMAP authentication / provisioning issue.