Go to yahoo email and "options" "pop forwarding" There it gives you the option to "not allow bulk mail or spam to be forwarded"...something like that. You will know it when you see it.
Good Luck. Go here: (check the "when downloading messages....."
POP Access & Forwarding [ My Billing Info - Need Help? ]
Web & POP Access [POP Settings]
Access Yahoo! Mail via the web interface and through any email program using POP3 (e.g., Eudora, Outlook, or Netscape Mail).
When downloading new messages:
I don't want to receive any messages that SpamGuard thinks are spam
I want to receive all messages, including those that SpamGuard thinks are spam
For messages that SpamGuard thinks are spam, add [Bulk] as a prefix to the subject
Learn more about these options - View POP Settings