SME 7:
server-manager:
-Configuration
--E-Mail
---Change E-Mail Delivery Settings
Plug in the IP address of the Exchange Server as the "Address of internal mail server", then reconfigure your firewall to send incoming SMTP traffic to the SME server.
Your options for handling "Spam" are slightly limited - you can't push it directly into the Exchange users' "Junk E-Mail" folders, and therefor some of the spam/ham training info on Contribs is irrelevant, but it does a great job of reducing SPAM.
If they load & enable the "Microsoft Intelligent Mail Filter" on the Exchange server, too, they should be able to approach postini in effectiveness.
Reporting is not as good, though. I believe that Postini provides a link for each user to a webmail-like page full of recent spam that gets cleared out weekly or so. I'd *love* to have that on my SME 7 box in conjunction with an internal Exchange server, but haven't figured it out yet!