One of the trends (might I say minimal requirements) in the hosted email filtering space today is that of email storage and archiving. Exchange Hosted Services does it, and a number of our competitors do as well.
Government regulations often spring up new businesses. When Sarbanes-Oxley passed in 2002 in the wake of all the corporate scandals in the United States, it became law to keep copies of all corporate communication. This includes email. As the implications of the law became more and more apparent to everyone, it became equally clear to anti-spam vendors that email archiving solutions were going to play an integral part in their service offerings.
Obviously (in my opinion), the best way to do email storage is via database backends. That makes it searchable, particularly if you have full text indexing on the content of the message. For large companies who are sometimes subject to litigation or discovery, the ability to hunt through email in real-time (or near real-time) is crucial. I would think that Microsoft has an advantage in this regards since one of their own in-house technologies is SQL server. Although, I guess other companies could get by with Oracle or MySQL but I don't know enough about performance evaluations to judge whether one is better than the other. I do know that Microsoft solutions are designed to integrate with each other (ie, run on Windows and use SQL Server).
Anyhow, one of the direct applications of the email archiving solution is in the resignation of New York State governor Eliot Spitzer. Apparently, they have been able to access email records of him engaging in illegal activities. With all of the legal requirements surrounding email archiving, authorities would be able to search through all his email records in order to build a case against him. This is an example of how technology has been able to assist in law enforcement.
Communications are a great thing; archiving communications comes with great responsibility.