Confirmed. Thread notification emails to your yahoo email address are soft bouncing; similar thread notifications to other users' yahoo mail accounts are not soft bouncing.

A hard bounce usually indicates a permanent mailbox error (like that user account no longer exists). A soft bounce usually indicates a temporary error (like a full mailbox, or an account over quota). So next step would be to double-check your Yahoo account and settings to see if there's anything there that catches your eye.