Better way to handle virus attacks

Where I work if there is high virus activity on the internet the system blocks all 3rd party email from outside the company and everything from yahoo or hotmail. Several of our clients use yahoo or hotmail accounts and communication becomes uncertain during these virus attacks. Is there a better way to handle this? I have been tasked with developing a better system to handle virus threats. Is there an easy solution to this? What kind of costs would be associated with implementation? Any advice would be welcome.