'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.'

def lookupDomain(self, user):

    try:
        (user, domain) = user.split(self.domainSpecifier, 1)
    except ValueError:
        domain = ''
    if (domain not in self.service.domains):
        raise pop3.POP3Error(('no    such    domain    %s' % domain))
    return (user, domain)
