Quickest email set up using Google Apps and Linode for SMTP

I will be using my Linode VPS to host a decent number of client websites, and offering Google Apps email services for any client's email needs.

I have configured the DNS to use Google Apps correctly and this is pretty straight forward, but I am having trouble getting a SMTP configuration working so that, when managing a collection of email address from a single Gmail account, it will not send 'On Behalf Of…'

The set up for this within Gmail requires SMTP authentication from my server.

Currently I am able to use PHPMailer on my server (does this mean it must already have Postfix, or similar, already installed? I used a StackScript to install Unbuntu 10.04.2 LTS), but still need assistance in configuring my server to handle this SMTP requirement.

I have looked at the tutorials here: http://library.linode.com/email/postfix/ but not sure if any of these are appropriate.

tl;dr: I do not require email features, Google Apps handles this, just need SMTP functionality to send mail from Gmail via Linode server.

Thanks in advance for any assistance