Who is carrierzone and what do they have to do with email through a custom domain?

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http://serverfault.com – I've got a friend who has his email set up with a custom domain, registered through Earthlink. He wants to change domain registrars. Earthlink simply hosts the domain and MX records which point to servers owned by "carrierzone". I've found some Google results that suggest carrierzone is connected with Earthlink. And carrierzone's website seems to say they're primarily some kind of spam filtering service. My friend hasn't heard of carrierzone before. I'm assuming they provide email hosting / SMTP service. I'm hoping that I can help him switch registrars and simply copy the MX records to (HowTos)