Email (Electronic Mail) System Basics
Email is a system for exchanging digital messages over the internet. The email system operates based on a structure of complex protocols and servers, far beyond simple message delivery.
1. Key Components of the Email System
The email system is primarily composed of three main elements:
① Mail User Agent (MUA)
The program or interface that users interact with to read, write, and manage their email. * Examples: Outlook, Gmail web interface, mobile mail apps.
② Mail Transfer Agent (MTA)
The server program responsible for transferring (relaying) email messages from one server to another. * Primary Protocol: Uses SMTP (Simple Mail Transfer Protocol) for sending and relaying mail.
③ Mail Delivery Agent (MDA)
Responsible for receiving the incoming mail and finally storing and managing it in the recipient's mailbox.
2. Mail Transmission and Retrieval Process (Protocol Roles)
The journey of an email from sender to recipient involves several protocol steps:
- Sending: The user (MUA) composes and sends the email. The MUA uses the SMTP protocol to send the message to the user's Outgoing MTA (Server).
- Relaying: The Outgoing MTA queries the DNS MX Record to find the domain's dedicated Receiving MTA (Server) and transfers the mail using SMTP.
- Receiving & Storing: The Receiving MTA accepts the mail and securely stores it in the recipient's Mailbox (MDA).
- Retrieval: When the recipient wants to check their mail using their MUA, the MUA connects to the server using the IMAP or POP3 protocol to fetch the messages.
3. Summary of Core Communication Protocols
| Protocol | Role | Direction | Characteristic |
|---|---|---|---|
| SMTP | Transfer (Sending & Relaying) | Client → Server, Server → Server | Push method, ensures reliable transfer (TCP-based) |
| IMAP | Retrieval & Management | Server → Client | Server synchronization, enables multi-device access |
| POP3 | Retrieval & Download | Server → Client | Local download, typically deletes mail from the server upon download |