'@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.'

def __init__(self, service, user, host=smtp.DNSNAME):

    self.service = service
    self.user = user
    self.host = host
