I am curious as to what the government archiving requirements actually are. Not needing the security of an archiving appliance, I find that SM's archiving and the compression / indexing it uses are close to ideal for my clients, although I haven't installed v16 anywhere, and don't know how that version archives.
There ought to be some appliance that acts as a gateway host and does that job. Check out Symantec's email cloud suite. It might be in there. I use their service for incoming/outgoing email anti-virus/spyware/malware etc. filtering, and it serves as a smarthost/gateway host, keeping most badware from even crossing into the companies' LANs.