Content-filtering software from third-party suppliers can be hooked in to your deployment through Messaging Server’s conversion channel. Channel keywords are used to enable mail filtering using anti-spam and anti-virus products, such as Brightmail or SpamAssassin. You can configure the MTA to filter for all messages or only those going from or to certain channels, or to set the granularity at a per-user level. A user can decide to use spam or virus filtering, or both. (SpamAssassin only filters for spam.)
An extensive Sieve support enables great flexibility to set the disposition of the message determined to be spam or virus. You can take the default action of discarding the virus and spam, or filing the spam into a special folder. But using Sieve, you can forward a copy of the message to some special account, add a custom header, or use the spamtest Sieve extension to take different action based on a rating returned by SpamAssassin.