First goto the cog on the right side of your Gmail window and select "Settings" from the menu.
When the settings open, select the "Accounts and Import" tab
Then select "Add Another Email Address..."
On the window that opens enter your name to be shown and your email address, click next (see image)
On the next window, enter "smtp.yourdomainname.com.au"
Then your full email address as the username
Then enter your password.
Set the port number to 25 using TLS
leave the port number and options as per the image example.
Click add account and its done.