Rafeeq is correct- SMSMSE is a "must have" for Exchange servers. It scans for threats, unwanted content, spam, and more. Unless the enterprise is protected by a dedicated mail security appliance, install SMSMSE on your mail servers.
The latest Symantec Intelligence Report has currnet info on the amount of threats which are in circulation.
http://www.symantec.com/content/en/us/enterprise/other_resources/b-intelligence_report_11-2014.en-us.pdf
Also, be sure end users are educated not to follow suspicious links and download unexpected attachments! That's a very common method why which malware gets into a company.
Hope this helps!
Mick