Simple SMTP Messages: Identifying the Client and the Recipient

Figure 8-6. Simple SMTP Messages: Identifying the Client and the Recipient

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The SMTP protocol defines messages so that the e-mail clients and servers can manage the e-mail forwarding process. The Extended Hello (EHLO) command identifies the client. The Mail command tells the server that the client wants to either send or receive e-mail. The RCPT (short for recipient) identifies the, well, uh, recipient of the e-mail that is about to be sent. After the server replies with an acknowledgement (ACK) message to each of these first three commands, the e-mail can be transmitted.

Note that when any of the devices actually sends the e-mail, that message conforms to the Internet Message Format (RFC 2822) specifications for the contents of the header and text. The SMTP RFC (RFC 2821) defines the messages that control the process with SMTP, such as EHLO and MAIL.