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2. How Internet Mail Works > Summary of the SMTP Protocol

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.[14] Replies always start with a three-digit decimal number; for example:

250 Message accepted

The text is usually information intended for human interpretation, though there are some exceptions, where the number encodes the type of response. The first digit is the most important, and is always one of those shown in Table 2-1.

Table 2-1. SMTP Response Codes
CodeMeaning
2xx The command was successful
3xx Additional data is required for the command
4xx The command suffered a temporary error

  

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