def lookupDomain(self, user):
        """
        Check whether a domain is among the virtual domains supported by the
        mail service.

        @type user: L{bytes}
        @param user: An email address.

        @rtype: 2-L{tuple} of (L{bytes}, L{bytes})
        @return: The local part and the domain part of the email address if the
            domain is supported.

        @raise POP3Error: When the domain is not supported by the mail service.
        """
        try:
            user, domain = user.split(self.domainSpecifier, 1)
        except ValueError:
            domain = b''
        if domain not in self.service.domains:
            raise pop3.POP3Error(
                "no such domain {}".format(domain.decode("utf-8")))
        return user, domain

        Check whether a domain is among the virtual domains supported by the
        mail service.

        @type user: L{bytes}
        @param user: An email address.

        @rtype: 2-L{tuple} of (L{bytes}, L{bytes})
        @return: The local part and the domain part of the email address if the
            domain is supported.

        @raise POP3Error: When the domain is not supported by the mail service.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/mail/protocols.py