Mailhop relay domain alias

I am currently looking at a new anti spam solution after the major issues I have been suffering with Postini (currently loosing about 25% of email). Mailhop services look like the best soultion compared to the other vendors as it is a great example of the KISS principle.

Before I migrate across there is only issue that is of concern. I have one primary domain with several aliases (The primary is http://domain.com with alias domains for .net, .com.au, .net.au) and I am wondering if I will need to buy a mailhop service for each domain or if I buy one domain and can configure the rest as aliases of some sort.