Suppose I want to subscribe to all activity in response to a question I am interested in. I am aware of the existence of an RSS feed, but I want email notifications. How do I do this?
There are services that will send the content of RSS feeds to your email inbox, for example, Feed Mailer. Of course that's not optimal, but it's a decent workaround.
You can't subscribe to a specific question made by someone else by email.
If it's your own question, you can get emails when new answers are posted using the link at the bottom of the "Ask Question" page or at the bottom of the page below all the answers for questions that have already been posted.
For your own questions you're also notified when someone comments on them, so you could have your inbox notifications emailed to you, but it's not per-post or limited to responses to your own questions.
Last but not least, there is no way to subscribe by email to all activities (edits, comments, etc.) even for your own posts.
As per this answer/comment (thx Yi Jiang's 独角兽), the email settings moved. There should be a link (it reads "email settings") in the bottom bar of the popup that appears when you press the Stack Exchange button (top-left of web page) that will take you to your email settings. In said email settings dialogue, maximum frequency of notification is every 3 hours (as of today, anyway).