Google’s Gmail service as added virus scanning and removal to its feature list, the website says it works like this:
Each time you send and receive attachments, Gmail automatically scans them for viruses.
If a virus is found in an attachment you’ve received, our system will attempt to remove it, or clean the file, so you can still access the information it contains. If the virus can’t be removed from the file, you won’t be able to download it.
If a virus is found in an attachment you’re trying to send, you won’t be able to send the message until you remove the attachment.”
I have yet to be able to test it but I do wonder how effective it is, where the definitions come from, and how often they are updated.