def __init__(self, service, user, host=smtp.DNSNAME):
        """
        @type service: L{MailService}
        @param service: A mail service.

        @type user: L{bytes} or L{None}
        @param user: The authenticated SMTP user.

        @type host: L{bytes}
        @param host: The hostname.
        """
        self.service = service
        self.user = user
        self.host = host

        @type service: L{MailService}
        @param service: A mail service.

        @type user: L{bytes} or L{None}
        @param user: The authenticated SMTP user.

        @type host: L{bytes}
        @param host: The hostname.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/mail/protocols.py