How to send mail notification about build failure in jenkins.. with example

Here I have used,
Plugin: Jenkins Mailer Plugin
Mail: Gmail
Code to be added in your Jenkinsfile:
post {
failure {
echo 'sending email notification from jenkins'
step([$class: 'Mailer',
notifyEveryUnstableBuild: true,
recipients: emailextrecipients([[$class: 'CulpritsRecipientProvider'],
[$class: 'RequesterRecipientProvider']])])
}