All messages released from the Spam section will include a header, indicating why we held the message. The information is in the X-Pobox-Antispam header.
A list of currently active reasons is:
country/XX returned deny (where XX is the two letter country code for the country that sent the message to us)
dnsbl/XX returned deny (where XX is the URL for the blacklist which contained the IP address that sent the message to us)
rhsbl/XX returned deny (where XX is the URL for the blacklist which contained the domain name that sent the message to us)
broadband/ returned deny (the IP that sent the mail looks like consumer broadband; these are primarily spambots running on compromised computers)
require_ptr/ returned deny (the sending IP address did not have a PTR record)
cloudmark/ returned deny (the message contained content currently found in the cloudmark spam filter)
fake_pobox_address/ returned deny (the message sender is an address that is not active in the Pobox system)
filter/ returned deny: user email filter (for messages discarded by your Email Filters)
check_infwds/ returned deny: you have told us you are forwarding mail from another domain; we checked the IP that sent the mail to that domain, instead of the IP that sent mail to us.