def exists(user):
        """
        Check whether a user exists in this domain.

        @type user: L{User}
        @param user: A user.

        @rtype: no-argument callable which returns L{IMessageSMTP} provider
        @return: A function which takes no arguments and returns a message
            receiver for the user.

        @raise SMTPBadRcpt: When the given user does not exist in this domain.
        """

        Check whether a user exists in this domain.

        @type user: L{User}
        @param user: A user.

        @rtype: no-argument callable which returns L{IMessageSMTP} provider
        @return: A function which takes no arguments and returns a message
            receiver for the user.

        @raise SMTPBadRcpt: When the given user does not exist in this domain.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/mail/interfaces.py