Getting "Domain Name Unavailable to Access" error

I have a client attempting to map a domain to my multisite network, and I'm getting a "Domain Name is Unavailable to Access" error on this.

Looking at, his DNS records look correct and are pointed to my server. He's got a TTL set to 600s, and running through a variety of WHOIS tools, his domain resolves to my IP address.

Running nslookup from my server's shell it shows my server's ip in the non-authoritative answer, and localhost ( as the server.

I'm running my network on MediaTemple, and have registered his domain as an alternate on my site, and I've created the symlinks (mt) requires for a mapped domain to work. Visiting his domain in a browser brings me to my network's wp-signup page.

The define('SUNRISE', on); line is in my wp-config.php file. Commenting out the 4 lines beginning with define( 'DOMAIN_CURRENT_SITE', 'mysite'); has no effect.

I'm lost here. Any suggestions of what I'm missing?