Hello UbuntuForums

I host a small website on a Ubuntu server 12.04
Lamp,ssh,mail,webmin-
Everything works fine,got a free hosting for a year.
For better access of the cms i got a Ubuntu server installation.

I have pointed they A Records on the DNS on hosting company to they local server ip.
It all works perfect, i manage to find a guide and make it work on gmail.

The mail server is free on the hosting company.

My question is:
How do i setup the Ubuntu server to relay and send to another host ex, mx.mailhost.com
I want visitor to send question straight from the site,

Thank you for your time