SEEN-BY
Lines at the end of an echomail message listing every node that had already received it, so nobody sent it there again.
Echomail spread by each node forwarding to its neighbours. With no memory, a post could circle a ring of nodes forever. Tossers prevented that by appending their own and their recipients' net/node numbers to the SEEN-BY lines and skipping anyone already listed. A hidden PATH line recorded the route actually taken.
The format had a flaw baked in: it held net and node, but no zone. Once FidoNet went global the same numbers existed on different continents, so SEEN-BY lines were stripped at zone gateways. That is one reason duplicate checking on message IDs remained necessary regardless.