A lot of our mail server bandwidth gets chocked by spam and virus. Postini is an excellent cloud based solution, which cleans our emails before they reach our network.
It is used by very large organisations like GE, Genpact, etc.

Postini includes:

• Email Security
  It filters out all messaging threats including spam and viruses before they reach your network.
  
  
• Email Archiving
  It manages email retention with a centralized, searchable archive by which administrators and users can locate email quickly.
  
  
• Web Security
  Stop web-borne spyware and viruses before they infiltrate your network and compromise or disable computers. All web requests are scanned in real-time, providing with dynamic, real-time and multi-layered protection.