E-Mail Notifications in StackOverflow
This option is found in the preferences section of the user profile. Additionally, how do you subscribe to a question or a tag?
If you "Allow email notifications" then that means you will receive them. (Once subscribed to questions or tags)
To subscribe to a tag, click on this link, which can also be found under your favourite/ignore tag lists on the main page. Follow the instructions there.
I can't see how to subscribe to an individual question.