Mail Transfer Agents (MTAs) and Mail User Agents (MUAs)
The role of a Mail Transfer Agent (MTA) is to transfer mail from one mail server to another. The schematic diagram for an email 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 email client program or similar name), the message moves into the sending SMTP server’s message queue, where the MTA (for example, Sendmail) pulls it out as soon as a connection to the receiving SMTP server becomes available and transmits it. (Because this process transmits a ...
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