a groovy script to test your email extension templates cf. https://wiki.jenkins-ci.org/display/JENKINS/Email-ext+pluginto do that just * replace the projectName and p.recipientList values in the script below* run this script through http://localhost:8080/jenkins/script ** copy and paste it** click "run"

debug-jenkins-email-ext-plugins.groovy

import hudson.model.StreamBuildListener
import hudson.plugins.emailext.ExtendedEmailPublisher
import java.io.ByteArrayOutputStream
   
def projectName = "your-project-name-here"
def oriProject=Jenkins.instance.getItem(projectName)
if(oriProject==null){
    println "oriProject is null, exit."
    return 1
}
Jenkins.instance.copy(Jenkins.instance.getItem(projectName), "$projectName-Testing"); 
     
def project = Jenkins.instance.getItem(projectName)
try {
  def testing = Jenkins.instance.getItem("$projectName-Testing")
  def build = project.lastBuild
  // or def build = project.lastFailedBuild
  // see the <a href="http://javadoc.jenkins-ci.org/hudson/model/Job.html#getLastBuild()" title="Job" target="_blank">javadoc for the Job class</a> 
  //for other ways to get builds
 
  def baos = new ByteArrayOutputStream()
  def listener = new StreamBuildListener(baos)
 
  testing.publishersList.each() { p ->
    println(p)
    if(p instanceof ExtendedEmailPublisher) {
      // modify the properties as necessary here
      p.recipientList = '[email protected]' // set the recipient list while testing
       
      // run the publisher
      p.perform((AbstractBuild<?,?>)build, null, listener)
      // print out the build log from ExtendedEmailPublisher
      println(new String( baos.toByteArray(), "UTF-8" ))
    }
  }
} finally {
  if(testing != null) {
    // cleanup the test job
    testing.delete()
  }
}