Here's how to report junk email to the sender's ISP. (An ISP is an Internet Service Provider, such as American Online, Earthlink or Yahoo.)
You should forward the email to abuse@isp.com, where isp.com is the ISP address, such as abuse@hotmail.com. In the case of America Online, the address to forward the spam to is tos@aol.com.
Yahoo suggests the following when reporting spam to them. These guidelines work for any ISP:
If you are reporting an abuse issue, please include the following to assist
us in a prompt and full evaluation of your report:
1) Original subject line. Please forward the email with a subject identical to
the original subject.
2) Complete headers. Email programs often display abbreviated headers. To
learn how to display the full headers in a Yahoo! Mail account, please visit our
Help Desk at:
http://help.yahoo.com/help/us/mail/config/config-11.html
If you are using a different client to read your email, please consult your
email program's help system for more information on viewing full headers.
(If you are using Microsoft Outlook, go View/Options. You will see the
Internet Headers box: Copy and paste that into the first part of your
forwarded message.)
3) Complete message body. Please include the complete, unedited content of the
email message in question. Please do not change or edit the message in any way.