For: Very powerful system especially targeted at large enterprises
Against: Can be hard to deploy, gets much of its strength from optional add-ons
Verdict: If you have a very large enterprise with complicated email requirements and a need for highly configurable email security, have a close look at this.

Mailstream Manager is intended to run in a few very specific Linux and Unix environments, including Red Hat ELAS, SuSE SLES and Solaris. The logic given to us by the developer was that large companies don't use Windows for their email systems.
Nonetheless, we noticed on the Sendmail website that a Windows version for Server 2003 is available. The vendor, however, did not provide that product to us for review.
The implementation is relatively straightforward if you are used to working with Unix or Linux. Once the software is installed, further configuration is conducted through a web interface. A good knowledge of the Unix/Linux command line is important for successful implementation.
We viewed Mailstream Manager largely as an email management platform. It has several optional hooks that allow a variety of anti-virus, anti-spam and other add-ons. It is only with these add-ons that the real power of the product can be realised.
Mailstream Manager sports a very powerful policy engine, which is not limited to security management of email. There are several operations-related functions, such as outbound content filtering.
The documentation is quite complete, if somewhat difficult to navigate. The manual seems to be another example of a product written by engineers for engineers so, again, a good knowledge of internet messaging helps significantly.
There are several support options, ranging from simple support during the work week to one-hour immediate 24/7 response for large enterprises. The website has some information, but for manuals etc. an ID and password are required.
Pricing is based upon annual subscriptions on a per-user basis. This tops out at around £3.40 (A$8.29) per user per year. Support plans range from 19 per cent to 30 per cent above the subscription price.