What is multistage open relay?
An open relay is not necessarily a single server, it can be a collection of servers as well. The open relay which consists of multiple servers is called a multistage open relay. In a two-stage example, the open relay consists of two servers. Call the first server “A” and the second “B”. The first server (“A”) accepts mails for relaying from everyone and relays them to server “B”, who “trusts” server “A”. In this case, it is server “B” which relays the mail to the final recipient. Server “A” is called the input relay and “B” is called the output relay. Be careful when filtering for output relays, because output relays can include innocent ISPs (even large ones).
