I believe the Yahoo! spam filter does not work. I handle multiple Yahoo! accounts via IMAP / SMTP. When I receive an email that should go to Inbox and goes to Spam, I move it from Spam to Inbox (via IMAP / SMTP). However, the algorithm is not learning from this behavior, and keeps landing the emails from that domain in spam. Any ideas why this is happening? Other ESPs (email service providers) learn from the user behavior.