I have made a simple sample post buld action in my Jenkinsfile for sending a relevant e-mail message for the status of the build although this is not being sent. 

post {        
   always {            
      emailext body: 'A Test EMail', recipientProviders: [[$class: 'DevelopersRecipientProvider'], [$class: 'RequesterRecipientProvider']], subject: 'Test'        
   }    
}

In the console ouptut, I observe that the only thing that has been logged is the "sending email to: <<email address>>"

Configuration with SMTP is completed as email message is being sent by putting the same post build action manually from UI.