If configured, Jenkins will send out an e-mail to the specified recipients when a certain important event occurs.

1. Every failed build triggers a new e-mail.
2. A successful build after a failed (or unstable) build triggers a new e-mail, indicating that a crisis is over.
3. An unstable build after a successful build triggers a new e-mail, indicating that there's a regression.
4. Unless configured, every unstable build triggers a new e-mail, indicating that regression is still there.

For lazy projects where unstable builds are the norm, Uncheck "Send e-mail for every unstable build".