SpamAssassin is free. Go to http//www.spamassassin.org for software and documentation.
SpamAssassin can be tuned and configured to provide very accurate detection of spam. The tuning is up to you and the SpamAssassin community. Messaging Server does not provide or enhance what SpamAssassin can do.
While no specific numbers are available, SpamAssassin seems to reduce throughput more than Brightmail.
SpamAssassin integrated with the MTA can be enabled for a user, a domain, or a channel.
SpamAssassin can be configured to use other online databases such as Vipul Razor or Distributed checksum clearinghouse (DCC).
Messaging Server does not supply an Secure Socket Layer (SSL) version of libspamass.so, however, it is possible to build SpamAssassin to use openSSL.
Perl 5.6 or later is required.
SpamAssassin can run on a separate system of its own, on the same system as the Messaging Server in a single system deployment, or on the same system as the MTA in a two-tier deployment. If Local Mail Transfer Protocol (LMTP) is used between the MTA and the message store, the filtering must be invoked from the MTA. It cannot be invoked from the message store. When SMTP is used between the MTA and the message store, it can be invoked from either one, and it can run on either system or a separate third system.
If you want to use a farm of servers running SpamAssassin, you would have to use a load balancer in front of them. The MTA is configured with only one address for the SpamAssassin server.