1) Email filtering can occur as an incoming gateway, embedded process, or outgoing gateway (including all 3). They tend to serve different purposes. Incoming priority is spam filtering, embedded filtering priority is breakout detection and containment (if spam filtering is done previously). Outgoing priority is Data Loss Prevention (DLP) and encryption (primarily web-based relay.) Declude lacks the features needed to be competitive as an outbound gateway. This limits its market potential, since any purchased outbound gateway product will offer inbound features as well.
2) It is possible to configure multiple incoming gateways in series, so you are not limited to the features of one product. I am currently using three gateways in series.
3) Most email filtering products are configured as incoming and outgoing gateways, with very different filtering configurations based on the flow direction. Increasingly, these products are cloud-based services, partly because of the success of Office365, and partly for other reasons. This also works against the Declude market potential.
4) From this forum, I have the impression that many Declude sites run embedded, since it works well that way. As long as you are willing to use a gateway configuration, the choice of email filtering product is completely independent of the email server product. Running an incoming gateway using SM v16 and Declude allows the mail server to upgrade SM independently of the Declude system.
5) Email filtering has two aspects: threat knowledge and the rules engine. I said that Declude is the best available product because of the strength of its rules engine and its extensibility. Declude relies on RBLs for threat knowledge, as it has no proprietary data source of its own. Bigger vendors will suggest that they have superior threat intelligence. This may or may not be true, as I have no way of knowing how to compare options. I am at minimum impressed by the number of RBL services that Declude can be configured to use. What I know for sure is that the email filtering marketplace is rife with products that have glaring omissions in their rules engine capabilities.