A discussion Mark [1] and I were having turned towards email—specifically, sending email to AOL (America On-Line) [2]. He's communicating with some people who use AOL (America On-Line) and is concerned about his email getting through. He heard that AOL had proposed some new DNS (Domain Name Service) record and was curious if I heard anything about it. I had not, but I started doing a Google search (AOL DNS record) [3].
Going through the results, I came across a link to the AOL Postmaster Info [4] page, a document that describes how AOL handles email and the criteria they use to accept or reject email. There is some very good information here (for all the ribbing AOL gets from the rest of the Internet, their infrastructure and networking architecture is incredible and they certainly know what they're doing technically, if only to handle the millions of subscribers).
[3] http://www.google.com/search?hl=en&ie=UTF-8&oe=UTF-8&q=AOL+DNS+record&btnG=Google+Search