client and server
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... SMTP service whereby an SMTP
client and server may interact to give the server an opportunity to
decline to accept a message (perhaps temporarily) based on the
client ...
... The remainder of this memo specifies how support for the extension
affects the behavior of an SMTP client and server.
...
... transmitting the entire message to
the server (which discards the transferred message). If, however,
both client and server support the Message Size Declaration service
...