How to configure user triggered external mail from webmail to use third party SMTP server

With reference to the title, I have the below goal.
i. All mails (system and user triggered) will be using an external smtp server.

I know I need to modify main.cfg in postfix. However, I am thinking if there's a way it can be configured from the UI ?
Just checking if anyone have attempted to override the auto generated main.cfg with your own predefined value?

 

Damm, thanks for pointing it out. However, I get it working by modifying main.cf.

 

Oops, is there a way I able to restore the original copy of it (in an automatic manner), so that I can configure it in ISPConfig?

 

You don't need to recover anything and nothing will stop working in the future.

I asume you manually added or changed "relayhost=" in /etc/postfix/main.cf?
And maybe "relayhost_user=" and "relayhost_password="?
Those settings have been overwritten in main.cf by ISPC when you changed the relay setting under System > Server Config > Email