Currently, the email-ext plugin sends emails as follows:
Improvement - An email will be sent any time there is an improvement. A build is considered to have improved whenever it has fewer failures than the previous build.
Fixed - An email will be sent when the build status changes from "Failure" or "Unstable" to "Successful".
However, there is an ovelap between these two conditions. If both triggers are configured, and a build changes from "failing tests" to "no failing tests", then two emails are sent.
Improvement should mean that the number of failing tests decreased, but some were still failing. "Fixed" should mean all tests were fixed.