Virtualmin + Untangle = Smart Host?

I’m wondering if this is feasible idea…

90% of the clients we support have their own MS Exchange server for email. 100% of those clients have SPAM issues. We’ve implemented various solutions over the years: UTM bundle on a WatchGuard firewall, install an Untangle based firewall appliance, install anti-spam software right on the Exchange server etc.

Lately we’ve been looking at offering a “hosted” anti-spam service. I imagine it something like this: We have two servers at a co-location facility. The first server runs Untangle and basically acts as a mail stream filter. Good mail gets passed to Virtualmin, bad mail gets rejected etc. Virtualmin basically just forwards all of the mail to our clients Exchange server - which is setup to only accept mail from our IP address etc.

What potential problems could you forsee with a solution such as this? Has anyone implemented a similar "fixed cost" solution that they can then resell per domain \ mailbox? Thanks in advance.

I have no experience with this although it does sound feasible.
Also have a look at this thread by webwzrd how he managed to reduce spam with greylisting/postgrey. the results are very positive