[Google Admin Console] Spam and virus filtering Google spam filtering is integrated into Google Apps, and spam is purged on a rolling 30-day schedule. Spam filtering is simply "on" by default, but you can use the Spam setting to enforce more stringent filtering of bulk mail or to set up approved senders to bypass Google’s spam filters. Your non-Gmail mailbox users can also manage spam with the Message Center and Quarantine Summary. For instructions on setting up the Message Center and Quarantine Summary for your users, see Set options for non-Gmail mailbox users. Google also has built-in virus checking. Most computer viruses are contained in executable files, so standard virus detectors scan messages for executable files that appear to be viruses. Google helps block viruses in the most direct possible way: by not allowing users to receive executable files (such as files ending in .exe) that could contain damaging executable code; even if they are sent in a compressed (.zip, .tar, .tgz, .taz, .z, .gz) format. For more details, see How Gmail spam and virus filtering differs from Postini. [Postini Admin Console] 
Spam and virus filtering