I want an outgoing mail server ONLY for multiple domains. I plan to use Postfix as that seems to be the easiest to setup being very new to Ubuntu/Linux.

The setup I plan to have are as follows:

1. I want to use virtual domain with postfix i.e. my multiple websites must be able to send an email from each their respective domains i.e. [email protected] is sent from my domain1.com website and [email protected] is sent from domain2.com website
2. This is an outgoing mail server only i.e. I don't want any returned (or otherwise) email sent to my postfix server.
3. Incoming mail is handled by Google Apps/Gmail and is already setup.
4. I already set my SPF recording to designate my mx records and postfix server ip as valid email servers i.e. "v=spf1 mx include:mydomain.com -all"

How can I achieve this? I'm frankly a little confused, so some help would be appreciated.

I attempted to follow these guides here, but it doesn't seem right (and it isn't clear what all the settings mean):

How to configure Postfix virtual domains http://www.sysdesign.ca/guides/postfix_virtual.html

Postfix Installation *.slicehost.com/2008/7/29/postfix-installation

Basic Postfix settings (main.cf) *.slicehost.com/2008/7/31/postfix-basic-settings-in-main-cf

I can only post one link, but those articles above can be found by replacing * with articles in the hyperlink.