How does Apple determine which ISP they support?

Apple has nothing to do with any ISP, as far as I know. If there are problems with your ISP, it's they you need to talk to.

Apple Mail will work with any ISP that uses industry standard mail protocols such as IMAP and POP.

Problems occur when using email providers like the old AOL proprietary mail.

Mail will filter most spam that gets through the ISP's filters so I don't know why you would have to drop Earthlink.

Apple supports the protocols for just about every combination of server I've come across. Some are easier to set up than others because they happen to use default security settings and ports that are well established. Apple is working on improving support for Microsoft's proprietary Exchange server according to the blurbs we get with the latest updates.

The fact that Copper.net says they are 100% compatible with Apple Mail tells me that you should probably check your Mail program for issues.

After taking a quick look at Copper.net's email server support page for Apple Mail I don't see any reason why those settings won't work with Apple Mail. They are old-school standard settings.
http://www.copper.net/kb/64-Email-Setup-Instructions---Mac-Mail/
The instructions are a little dated for Tiger, but still resembles an account setup in Leopard.

Of course, I can't actually set up an account with them, but I have no doubt it's a pretty straight-forward process. I've never had Thunderbird work, and Mail not work at the same ISP. Sometimes it's a little tricky for ports and SSL stuff, but it can be figured out. In this case it doesn't look like there's anything out of the ordinary with their server settings.

If Copper.net is so well regarded by somebody, then there must be other users here that have either had success or failure with their email system.

birdy176 wrote:
When making the switch to Apple my old ISP did not work with my iMac OS X, Leopard

I have to say that sounds so strange. Using wireless, I move my Mac laptop from home DSL to a friend's cable modem and I get Internet, I go to coffee shops and libraries and I get Internet, I go to a different city and I get Internet, I go to different countries and I get Internet on my Mac. I don't even know the names of most of the ISPs I connect through simply by using public wireless networks all over the city, country, and world, and yet I rarely need to reconfigure the machine. Macs just use regular standard Ethernet and wireless Internet. Apple Mail just uses regular mail protocols. My Apple Mail checks my ISP mail and multiple Gmail accounts over IMAP.

In many cases a "problem with Macs" isn't because of some incompatibility with Macs, it's that you're dealing with someone who either is using something other than network standards, isn't aware how standard Macs are, or doesn't have a proper understanding of the standards in the first place.

Thanks for the feedback. Good computing.