Self Hosted Exchange to Exchange/Postini Hybrid Consulation

We're a small business with a small budget and we're currently hosting SBS 2003 internally. We have our e-mail delivered to our Google Apps account where we then download the e-mail to Exchange via POP so that there are no lost e-mails if the Exchange server goes offline. We've heard that you can use Postini as a smarthost instead. This would be a much better setup that using POP but we need consultation and implementation assistance.