Long answer: the message has too many Received: message headers.A received header is added whenever Postfix (or any MTA) receivesa message. A large number of Received: message headersis an indication that mail is looping around.
Side comment: email uses the opposite of the technique that is usedto avoid IP forwarding loops. With IP, the sender sets a TTL (timeto live) field in the IP header. The field is decremented by eachrouter. When the TTL reaches zero the packet is discarded and anICMP error message is returned to the sender....more