In this great article by Christopher Knight, "Is EarthLink Dropping Your Permission-Based List Members?" you can learn more about the recently problems you may have been having with Earthlink email addresses suddenly bouncing from your mailing list.
When you see a lot of bounces from a certain ISP (like Earthlink, Comcast, or others), it's a good idea to do a little manual research to find out what error message is really coming back from those ISPs. In Christopher's case, the automated message he was receiving back was a request that HE fill out a request for the recipient to receive his email. What looked like "hard bounces" (email addresses that were no longer valid) were actually being captured by Earthlink's challenge/response system.
This article is well worth reading if you send out email newsletters or other email marketing.
