code: func (srv *Server) ListenAndServe(addr string) error {
  
  	srv.configureDefaults()
  
  	l, err := net.Listen("tcp", addr)
  	if err != nil {
  		return err
  	}
  
  	return srv.Serve(l)
  }
docs: // ListenAndServe starts the SMTP server and listens on the address provided