If you're looking for a scalable e-mail solution, Proofpoint's e-mail security services and solutions may be your best bet. Proofpoint's advanced, rich feature set offers enterprise-class scanning to medium and large-sized organizations. The product is consistent across all avenues of deployment, whether it be hosted, purchased as an appliance, or installed on your physical/virtual server. A company that whole-heartedly supports application deployment in the virtual sense can be a very valuable partner in today's businesses.
Proofpoint acts as a "man-in-the-middle," accepting e-mails from servers around the world into their solution. A myriad of events can be performed on the e-mail, such as stripping or banning attachments, monitoring for keywords, scanning for malware and verifying that your e-mail server actually has an account for the recipient. After scanning, e-mails are delivered to your e-mail servers for retrieval.
I approached Proofpoint about their hosted service for three distinct reasons. First, I don't have to purchase, maintain and update any hardware. If I add 10,000 users to the solution, it's as simple as a phone call. Second, Proofpoint will queue your mail in the event of an e-mail server outage. While most outages result in bounce messages that can delay mail for days, Proofpoint's queueing insures that all messages are delivered immediately upon restoration of services. Third, Proofpoint does all my disaster recovery planning for me. With data centers in various locations around the world (you can limit your data to a particular geographic area, if that's a concern), a major power or data outage in an wide area of the country ensures that their services never stop receiving and scanning your organization's e-mails.