Yahoo detector scans too much of the message.
Current Yahoo bounces have text like
Sorry, we were unable to deliver your message to the following address.
<bogus-address@example.net>:
550: 5.1.1 <bogus-address@example.net>: Recipient address rejected: User unknown in local recipient table
---------- Forwarded message ----------
followed by the original message. The detector doesn't stop looking on the ---------- Forwarded message ----------
line and can detect additional addresses from the body of the original message.