def receivedHeader(helo, origin, recipients):
        """
        Generate the Received header for a message.

        @type helo: 2-L{tuple} of L{bytes} and L{bytes}.
        @param helo: The argument to the HELO command and the client's IP
        address.

        @type origin: L{Address}
        @param origin: The address the message is from

        @type recipients: L{list} of L{User}
        @param recipients: A list of the addresses for which this message
        is bound.

        @rtype: L{bytes}
        @return: The full C{"Received"} header string.
        """

        Generate the Received header for a message.

        @type helo: 2-L{tuple} of L{bytes} and L{bytes}.
        @param helo: The argument to the HELO command and the client's IP
        address.

        @type origin: L{Address}
        @param origin: The address the message is from

        @type recipients: L{list} of L{User}
        @param recipients: A list of the addresses for which this message
        is bound.

        @rtype: L{bytes}
        @return: The full C{"Received"} header string.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/mail/interfaces.py