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http://security.yahoo.com/password_scams.html#report

Report a Phishing Email
If you have received an email that says it's from Yahoo! or another trusted third-party, but you suspect it might really be a phishing email, please forward it to mail-spoof@cc.yahoo-inc.com. Please include the full e-mail headers (click here to learn how to view full headers in Yahoo! Mail).

Report a Phishing Web Site
If you know about a web site that is impersonating Yahoo!, or a web site Yahoo! hosts that is impersonating another trusted third party, please report it to us here.

If you know about a web site that is impersonating a company other than Yahoo! (for example, eBay, PayPal, a bank or a financial institution) which isn't hosted by Yahoo!, we encourage you to report the web site to the impersonated company directly. You might also want to report the phishing web site to the Federal Trade Commission (FTC) at the email address: spam@uce.gov.