In my experience, passing thru smeserver for web browsing does not have rock solid protection from Virus/Malwares. Its ClamAV does check for viruses. However, I have one company before which I use SME as Internet Gateway. I don't usually make any customization, aside from needed specifics. Smeserver seems to be stable even if clients encounter problem with viruses. I guess that's the marking of Linux. It just keeps on working, but some were able to reach end-users.