From: DeskNow Knowledge Base
http://www.desknow.com/kb/

Why was this message classified as spam?
http://www.desknow.com/kb/idx/4/142/article/
To understand why a particular message was or was not classified as spam, check the message Headers. DeskNow adds the following headers to all the messages that it analyzes:

X-DN-Spam-Blacklisted-By: xyz - this header appears if an external DNSBL service has blacklisted the server that sent this message

X-DN-Spam-Probability: x - this is the spam probability as calculated by the Bayesan tatistical analysis (if the message was blacklisted, the probability is implicitly 1)

X-DN-Spam-Blacklisted-By-SURBL: x the domain or internet address x is blacklisted by the SURBL service

X-DN-Spoofed-From-Address: YES - the message fails the antispoof check on the From header, enabled in Administration/Antispam/Address spoofing. In other words, the message claims a 'From' address that is a local domain (i.e. a domain defined in your DeskNow server), but it did not arrive from an IP authorized to relay, or from an authenticated smtp sender.

X-DN-Sender-In-Whitelist: YES - this header is added if the sender belongs to your whitelist or contacts (see below)

X-Spam-Flag: YES - this header is added if DeskNow has classified the message as spam