From the perspective of Messaging Server, anti-spam solutions operate in much the same way:
Messaging Server sends a copy of a message to the spam filtering software.
The spam filtering software analyzes the message and returns a verdict of spam or not spam. Some programs, like SpamAssassin may also return a spam score, which is a numerical rating of the probability of the message being spam.
Messaging Server reads the verdict and takes a Sieve action on the message (see 14.2.3 Specifying Actions to Perform on Spam Messages).
Spam filtering programs interact with the MTA through a protocol. The protocol may be a standard as in ICAP-based programs such as Symantec AntiVirus Scan Engine, proprietary as in Brightmail, or simply non-standard as in SpamAssassin. Each protocol requires software hooks to interface with the MTA. Brightmail and SpamAssassin were the first two spam filtering programs that could be integrated with messaging server. The MTA now supports the programs that use ICAP.