Tried manual plugin installation-----Didnot work
Tried manage jenkins>Available>--------Update did not work
Tried uploading plugin manually by advance setting---Did not work
Restarted jenkins after every attempt.....plugin Email-ext is not installed........
Jenkins needs Internet access to download plugins and updates. It is problem with your inernet settings.
configure proxy
https://www.safaribooksonline.com/library/view/jenkins-the-definitive/9781449311155/ch04s09.html