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Lesson 3 Email agents
ObjectiveDefine the roles of the three email system agents.
Moving email from sender to receiver requires intermediate stops. An agent[1] at each stop inspects the mail, and decides how to deliver it. Ultimately, an agent's job is to take the email it's given and move it one step closer to the recipient.

Mail User Agents

The first agent a user encounters is the Mail User Agent (MUA). This agent is responsible for providing email access to the user. An MUA might provide a graphical user interface or a simple text interface to the user. Within this interface, the MUA allows users to compose, sort, save, and print their email.

Mail Delivery Agents

MUAs communicate directly with Mail Delivery Agents (MDA). MDAs are local agents, responsible for all the email within a domain. User MUAs send and receive email from their MDAs via protocols such as POP3 and IMAP.

Mail Transfer Agents

MDAs then communicate with Mail Transfer Agents (MTA). These agents move mail across the entire Internet. When an MTA doesn't know where the recipient's domain is, it will send the email to another MTA for processing. When the MTA does know where the recipient's domain is, it will send the email directly to the domain's MDA for processing.
The image below clarifies the relationship between MUAs, MDAs, and MTAs. The arrows show how an email propagates through an MUA, into an MDA, through several MTAs, and finally back into an MDA for distribution.
Three Email - Agents

email Agents - Exercise

Before you move to the next lesson, click the Exercise link below to practice matching email agent roles to their definitions.
email Agents - Exercise
The next lesson describes the purpose of Linux MTAs and MDAs.
[1]Agent: A program in the email system that inspects an email and decides how to move it further toward the ultimate destination.
Mail User Agents (MUA): Programs responsible for displaying and composing email.
Mail Delivery Agents (MDA) The email agents responsible for local network email delivery
Mail Transfer Agents (MTA) : The email agents responsible for Internet-wide email delivery.
POP3 : An acronym for Post Office Protocol, version 3, POP3 is an older protocol that enabled MUAs to send and receive mail from MDAs.
Internet Message Access Protocol (IMAP): A standard protocol for accessing mail from your local MDA.