Mail Transfer Agents (MTAs) and Mail User Agents (MUAs)
The schematic diagram for an e-mail message's path from one person to another is shown in Figure 25.1. Whether the user is on a dial-up connection from a remote site or sitting at the console on the mail server itself, the path the message takes is the same: From the user's input into the Mail User Agent (commonly known as an e-mail client program), the message moves into the sending SMTP server's message queue, where the Mail Transfer Agent (MTA) (for example, Sendmail) pulls it out as soon as a connection to the receiving SMTP server becomes available and transmits it. (Since this process transmits a message that doesn't originate or terminate on the SMTP server machine itself, this step ...
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