The biggest caveat is that it only works with major email providers. Think Apple, Gmail, Yahoo!, Hotmail, Mail.com, and a few others. You can't use it to check the settings of your work email, for instance, not even if your employer uses Gmail on the back end, at least not with my testing.
That out of the way, it's stupid-easy to use. Load the page and get this form:
Mail Settings Lookup Landing Page
Enter your email address and note that Apple says it will not store that address. Hit the blue arrow, and you'll get settings for your incoming email server and outgoing server, too. You can use that information to set up an account in an email client.
Sample Mail Settings
*Thanks to Duane Straub for the tip!