SCAM/SPAM/ What to do against those problems

Yahoo! has no control over activities outside its service, and therefore
we cannot take action. You may try contacting the sender's email
provider, by identifying the sender's domain and contacting the
administrator of that domain. The sender's provider should be in a
better position to take appropriate action against the sender's account.

The email message itself does contain some information relating to the
sender's identity. Yahoo! includes the originating Internet Protocol
(IP) address in the full Internet headers of all messages sent through
Yahoo! Mail, so that we will have information regarding the origin of
messages sent through our system. The originating IP address should be
located in the very last "Received" line of the full Internet headers
and corresponds to the sender's Internet Service Provider (ISP).

Please see the following URL for more assistance:

http://help.yahoo.com/l/us/yahoo/mail/o ... use-17.htm

Once you have identified the IP address, you can conduct an IP lookup to
determine which ISP provides this person with Internet access. One such
lookup tool you may want to try is:

http://www.arin.net/whois/

You can then attempt to contact that ISP to report any abuse activities
occurring within their service.

In addition, please visit the following website for useful tools to
combat spam:

http://antispam.yahoo.com/
Spamming
What is spam?
Also known as UCE (unsolicited commercial email), spam is unsolicited email. Often, spam will relate to dishonest get-rich-quick schemes, bogus products, or invitations to pornography sites. Sometimes, spam mail contains viruses as attachments. For a more detailed explanation of spam and why it should be stopped, see the article on spam About the Problem by CAUCE (the Coalition Against Unsolicited Commercial Email).

Can I stop spam?
Prevention is better than cure. Be careful about giving your email address when filling in forms online. You could also consider using a separate email address for some public activities, such as chat rooms, in order to protect your main address from spammers.

What can I do if I am being spammed?
Do not write back to the spammer. Often, spammers will try to guess email addresses. If they get a reply from you it simply confirms that they have found a valid email address. Although many spam mails include a link for removal from their list, this is normally a trick.

If you wish to do something about spam you may have received, you should report it to the spammer's ISP. Most ISPs realize it is bad for their reputation to have this activity on their networks and should be able to take action against the spammer.

How do I find the spammer's ISP?
http://www.apnic.net/apnic-info/whois_s ... g/spamming

You need to open up the email header and find the spammer's IP address.