First, take a look at the rejection message you received from the recipient's mail server. If their administrator has followed our instructions, the message will contain a phrase like this:
*** SPAM Blocked from IP ADDRESS- For information: http://bl.csma.biz/
- For removal: http://bl.csma.biz/cgi-bin/listing.cgi?ip=IPADDRESS
If not, you can try to look at your e-mail application's outbound SMTP server setting (assuming SMTP is your outbound mail transport) and try looking up that host's IP address. Most larger ISPs use more than one server to gateway mail so this isn't necessarily going to give you good information.
In any case, if you're still confused you should contact your ISP. They know their e-mail infrastructure and can troubleshoot this much easier than we can.
I don't understand how to determine the IP address that's blocked.