Emails will go out - for stable releases. Obviously pre-1.0 is a bit of a different situation, but most users shouldn't be upgrading to non-stable releases, so sending out an email isn't really the best idea. Users that are active are the more general target for non-stable releases.
The emails that go out won't be instant either. Security announcements would probably go out more quickly, but I'd expect to see a delay between a maintenance release becoming available and an email being sent out to customers.