List emails of clients accounts on Google Apps

Question

You will have to excuse my confusion here...

Basically, we have clients with hosted accounts and I need to somehow get a list of emails per domain name.

Say, test.com is a Google hosted account and that it uses Google's email service for admin@test.com, and several more, how can I find (programmaticaly) this complete list of emails?

I've looked around for a while and it only brought more confusion. I think my question relates to this Google API document.

While I'm usually armed with at least one line of code, I'm really clueless on this issue. Perhaps someone out there might shed light on this issue.

PS: I also tried Google API Explorer, but it didn't help at all.