Strongmail reports that AOL is filtering email based on your hard bounces. The basic idea is if you’ve got way too many hard bounces, you’ve got bad list hygiene, and they don’t want you sending email to their servers.
If you’re using MailChimp’s managed lists, we automagically clean hard bounces from your list immediately, to prevent this sort of thing. If you manage your list by hand in some excel file, and you manually remove bounces and unsubscribes "whenever you can get to them," you’re going to have problems (and not just with AOL). If you’re sending your very first email campaign to an old list you’ve been collecting for years, you should remove any contacts older than 1 year, and then send your campaign in small chunks.
On a related note, here are "average email bounce rates by industry."
Also, the new MailChimp reports will show you what your bounce rate is by ISP: