Your mail has been rejected because it has been sent direct from a mailserver running on a dialup or other dynamic IP listed in the PDL (Pan-Am Dialup List) blocklist at http://www.pan-am.ca/pdl/
We block mail from known dialup pools and other dynamic IP blocks, due to the ever present risk of "direct to MX" spam originating from spammers running bulkmailer software equipped with smtp engine capablities, and "remailer" virii such as Sircam and Klez, which have their own smtp engines to remail copies of
Please set your mailserver up so that it sends out mail through your ISP's mailserver, or through any other mailserver with a static IP address that you have access to. http://www.mail-abuse.org/dul/gateways.htm
is an excellent
resource which lists the steps required to set up a "smarthost" in several mailservers.