Dynamically update whitelist

We currently have a 3rd party managed Astaro 220 and are having difficulty achieving the setup we want, and I don't trust the third party's ability to perform the due diligence in attempting to solve our issue.

We currently run a small exchange server in our head office for approximately 30 employees, and with our current mail provider we simply forward the emails from our top level domain email addresses to email@exch.domain.com. Our Astaro is configured to allow access on port 25 exclusively to the current providers mail servers.

Here's where it gets difficult. We are migrating the entire company (not just head office) to the Google Apps platform but would still like to maintain the small exchange server internally. We do not want just anyone to be able to mail the exchange server directly so keeping the current whitelist policy would be desirable, and simply continue to forward the emails to the exch.domain.com email addresses from Google.

The problem is, Google has a large range of IP's that can potentially change at any time, but can be easily found by querying Google's netblocks using the following nslookup: Is there any possible way for the 220 to poll the Google netblocks and dynamically update the whitelist? Or if there are any other suggestions I would be open to hearing them.