EMSI
The handshake that let two FidoNet mailers introduce themselves, agree on protocols and show passwords in a single packet rather than a long ritual.
Electronic Mail Standard Identification was described in FSC-0056 and developed around 1991, with FrontDoor author Joaquim Homrighausen among its authors. Earlier handshakes such as FTS-0001 and WaZOO carried little information per step. With EMSI the caller sent **EMSI_INQ, the other side answered, and then an EMSI_DAT packet went across carrying addresses, passwords, system name and the transfer protocols — ZMODEM, for instance — each end supported.
Because the string was easy to spot, a mailer could tell another node from a human within seconds. A variant, IEMSI, let terminal programs and offline readers log in to a BBS automatically. To modern eyes it looks like a binary protocol wearing a text costume, which is roughly what it was: it had to survive lines that could not be trusted with the eighth bit.