Know the Email Address for Spam Complaints
Generally, you can assume that
- abuse@isp.com
has been set up specifically to deal with spam complaints.
If you get spam from a Yahoo! Mail account, you should direct your complaint to abuse@yahoo.com, for example.
If Abuse@* Does Not Work
Some hosts received so much spam at the standard abuse address that they set up special mailboxes. The Network Abuse Clearinghouse keeps a database of such addresses which you can search easily.
How to Find the Correct ISP to Receive Your Complaint
To find out which ISP the spammer is using, it's best to use a service like SpamCop.

