Has nothing to do with ASSP or external mail servers. Your IP seems to be blacklisted at the remote Exchange server for whatever reason. Read http://support.microsoft.com/kb/928123
as advised and you'll see:
This issue occurs if the IP Block list is enabled on the Edge server in the receiving Exchange 2007 organization.
To resolve this issue, contact the receiving organization. Then, request that it add your mail server's IP address to the IP Allow list of the Exchange 2007 server that has IP address-blocking enabled. This action will make sure that anti-spam filters are bypassed for e-mail messages that are sent from your mail server.
In case you're using a dynamic IP, try using the SMTP of your ISP.