Email Wiki: SMTP, IMAP and POP3: Email Protocols Explained

Email systems rely on three core protocols for sending, receiving, and managing messages: SMTP (Simple Mail Transfer Protocol), POP3 (Post Office Protocol version 3), and IMAP (Internet Message Access Protocol). SMTP handles mail transfer, while POP3 and IMAP are used for mail retrieval. Together, these three protocols form the foundation of modern email communication.

Protocol Definitions and Functions #

SMTP (Simple Mail Transfer Protocol)

Purpose: Specifically designed for sending emails. It’s a “push” protocol that transfers email from the sender’s client or server to the recipient’s mail server. Process: User composes and sends an email via mail client (e.g., Outlook) → Client submits the email to the configured SMTP server using SMTP protocol → The server looks up the recipient’s domain mail exchange (MX) records via DNS → Establishes an SMTP connection with the target mail server and “pushes” the email to it. Ports: Default port 25 (unencrypted), recommended ports are 587 (with STARTTLS encryption) or 465 (SMTPS, SSL/TLS encryption). Characteristics: Only handles the email sending process, not involved in email storage or management.

POP3 (Post Office Protocol version 3)

Purpose: Specifically designed for receiving emails. It’s a “pull” protocol that allows clients to download emails from the mail server to local devices (such as computers, phones). Working Mode: Client connects to POP3 server. Default behavior: Downloads all new emails to the local device, typically deleting them from the server afterward (can be configured to keep copies). All email management (moving, marking, deleting) occurs on the local device, and operations are not synchronized back to the server. Ports: Default port 110 (unencrypted), encrypted port 995 (POP3S, SSL/TLS encryption). Characteristics: Simple design, primarily solves offline email access issues. Emails are ultimately stored on a single local device, reducing server storage usage.

IMAP (Internet Message Access Protocol)

Purpose: Designed for receiving and managing emails. It’s also a “pull” protocol, but its core is providing two-way synchronized access to emails stored on the server. Working Mode: Client connects to IMAP server. Emails always remain stored on the server; clients default to downloading only header information (subject, sender, time, etc.) or downloading email content as needed. Any operations performed by users in the client (reading, deleting, moving emails, marking status as “read/unread”, creating folders, etc.) are synchronized back to the server in real-time. Allows multiple clients (such as computers, phones, webmail) to access simultaneously while maintaining consistent state. Ports: Default port 143 (unencrypted), encrypted port 993 (IMAPS, SSL/TLS encryption). Characteristics: Provides rich email management features, supports seamless synchronization across multiple devices and remote access, with email status and organizational structure centrally stored on the server.

Similarities #

Core Objectives: All three are application layer protocols serving email systems. Collaborative Work: Sending email must use SMTP; receiving email uses either POP3 or IMAP, the two cannot be used simultaneously for sending. Security Evolution: All support encrypted transmission via SSL/TLS (corresponding to ports 465/587, 995, 993). TCP/IP Dependency: All run on top of the TCP/IP protocol stack.

Differences #