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This is [status-completed]!

I've added a few different types of email subscriptions:

 * Everything: get all the new questions. You'll get some duplicates, because this is just a reverse chronological feed.
 * "Interesting": questions with a score over a certain threshold (currently 1, but configurable and probably to be increased as good posts start to get voted up more), randomly ordered so you get something new each time.
 * Tag: reverse-chronology of a single tag.
 * User: reverse-chronology of a single user.

This also goes some way to addressing [this other feature-request](https://writing.codidact.com/questions/39317): that's still on my list because it should be UI functionality as well, but the Interesting emails serve a similar purpose.