def authenticateUserPASS(self, user, password):
        """
        Perform authentication for a username/password login.

        Override the default lookup scheme to allow virtual domains.

        @type user: L{bytes}
        @param user: The name of the user attempting to log in.

        @type password: L{bytes}
        @param password: The password to authenticate with.

        @rtype: L{Deferred} which successfully results in 3-L{tuple} of
            (L{IMailbox <pop3.IMailbox>}, L{IMailbox <pop3.IMailbox>}
            provider, no-argument callable)
        @return: A deferred which fires when authentication is complete.
            If successful, it returns an L{IMailbox <pop3.IMailbox>} interface,
            a mailbox and a logout function. If authentication fails, the
            deferred fails with an L{UnauthorizedLogin
            <twisted.cred.error.UnauthorizedLogin>} error.
        """
        user, domain = self.lookupDomain(user)
        try:
            portal = self.service.lookupPortal(domain)
        except KeyError:
            return defer.fail(UnauthorizedLogin())
        else:
            return portal.login(
                UsernamePassword(user, password),
                None,
                pop3.IMailbox
            )

        Perform authentication for a username/password login.

        Override the default lookup scheme to allow virtual domains.

        @type user: L{bytes}
        @param user: The name of the user attempting to log in.

        @type password: L{bytes}
        @param password: The password to authenticate with.

        @rtype: L{Deferred} which successfully results in 3-L{tuple} of
            (L{IMailbox <pop3.IMailbox>}, L{IMailbox <pop3.IMailbox>}
            provider, no-argument callable)
        @return: A deferred which fires when authentication is complete.
            If successful, it returns an L{IMailbox <pop3.IMailbox>} interface,
            a mailbox and a logout function. If authentication fails, the
            deferred fails with an L{UnauthorizedLogin
            <twisted.cred.error.UnauthorizedLogin>} error.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/mail/protocols.py