Today I received a spam that the headers clearly showed was generated within Yahoo and went directly from their mail system to mine. So I reported it to their published Abuse address, so that Yahoo would know their user is spamming. I received back the following e-mail:
This is an automated response; please do not reply to this email as replies will not be answered.
To report spam, security, or abuse-related issues involving Yahoo!'s services, please go to http://abuse.yahoo.com.
Yahoo! Customer Care
Fail #1: They are required to accept abuse reports at their published Abuse address.
Fail #2: Going to this address gets redirected to http://help.yahoo.com/abuse/ which has hundreds of different links, but after spending 30 minutes looking through every single one of them not a single one provides a place to report a spam sent by Yahoo.
Result: Yahoo no longer accepts spam reports. I am therefore blocking Yahoo on every mail gateway for which I have control, and listing them in the Pink Providers blacklist effective immediately.