thanks for reply. As i said i am working for an ISP. I set up mail server with fedroa 16+Dovecot. if our clients wants to send mails, i want to give them that it requires authentication by using username and password (Whoever is using mail clients like outlook, thunderbird). And as well as who ever is using mail servers they need to use my mail server as smarthost with username & password authentication.
so that i can open all networks and give authentication to users to use whole world.
Pelase help me how to set up this. i tried by googling but failed.