Sometimes it is handy to have a list of the top MAGY (Microsoft, AOL, Google, Yahoo) email domains. Here's what I've compiled based on current and prior research, plus input from readers.
AOL and Yahoo are merging, but I've left them distinct for now. Also, some of these domains might not be common email domains, but they all seem to have valid MX records and thus if you're looking for a list of domains to, for example, exempt from validity checking so that they never falsely appear to be dead domains, it'll still come in handy.
If you use this to group domains to count sending mail volume by ISP, it will be slightly imperfect, as I've included the corporate domains (microsoft.com, google.com, teamaol.com, oath.com, yahoo-inc.com) that may use separate mail systems. However, your send volume to those domains is probably pretty low compared to the consumer mailboxes, so for "back of napkin" kind of math in a pinch, it might do just fine.
Note that the intent here is to list email domains owned and used by that given provider; not to list every possible customer domain that points at the ISP's mail servers. I'm not capturing Google Apps domains, Yahoo Business Hosting domains, or anything like that. That's out of scope for this exercise.