I have login as admin
click on server setting

Mailman Settings is greyed out in the plesk cp > server settings.

Please tell me how to enable it?

waitng for reply...

 

if you have already setup mailman once, that is, you have selected mailman settings previously ... than it is supposed to be greyed out, that is the intended behaviour ... go into a domains mail and select the mail list tab to add a mailist ...

if you have not did the above, it may be that mailman is not installed, that was the case for me ... I ran the updater to get mailman installed ...

Good Luck !!!