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Chapter 2. How Internet Mail Works > Summary of the SMTP Protocol
2.5. Summary of the SMTP Protocol
SMTP is a simple command-reply protocol. The client host sends a command to the server, and then waits for a reply before proceeding to the next command.[10] Replies always start with a three-digit decimal number; for example:
[10] There is an optional optimization called "pipelining," which allows batches of commands to be sent, and batches of replies to be received, but this is purely to improve performance. The overall behavior remains the same, and we describe only the simple case here.
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