Common Email Protocols Explained: SMTP, POP3, IMAP and Modern Extensions

Published on 2025-04-20

10 min read

ChatTempMail Team

Overview of Email Protocols

Email, as one of the most fundamental communication services on the internet, relies on several standardized protocols to ensure cross-platform and cross-provider interoperability. These protocols define the format, transmission methods, and storage rules for emails, forming the infrastructure of the global email system. This article will detail the main email protocols and their working principles.

SMTP: Simple Mail Transfer Protocol

Basic Principles and Functions

SMTP (Simple Mail Transfer Protocol) is the standard protocol for transmitting emails from the sender to the recipient's server. It was initially defined in RFC 821 in 1982 and has undergone multiple updates since. SMTP's primary responsibilities include:

Establishing connections between the sending mail client and mail server
Implementing email relay and routing between mail servers
Managing error notifications and status codes during the email sending process SMTP operates at the application layer of the TCP/IP protocol stack and uses TCP port 25 by default. In the basic email architecture, SMTP is only responsible for sending and transferring emails, not for their final reception and storage.

SMTP Interaction Flow

A typical SMTP session includes the following steps:

Connection Establishment: The client establishes a TCP connection with the server
Greeting: The server sends a welcome message, including its identity and supported extensions
Authentication: The client provides identity credentials (in scenarios requiring authentication)
Sender Setup: The client specifies the sender using the MAIL FROM command
Recipient Setup: The client specifies one or more recipients using the RCPT TO command
Data Transfer: The client sends the DATA command, then transfers the email content
End Marker: The email content ends with a period (.) on a separate line
Server Confirmation: The server confirms receipt and returns a status code
Session Termination: The client sends the QUIT command to end the session

Modern SMTP Extensions

As the internet has evolved, the basic SMTP protocol has been enhanced with various extensions:

ESMTP (Extended SMTP): Adds new commands and options, supporting authentication, encryption, and other functions
SMTP-AUTH: Provides user authentication mechanisms to prevent unauthorized email sending
STARTTLS: Supports using TLS encryption on SMTP connections to protect communication content
SMTPS: SMTP using SSL/TLS encryption throughout the entire session
SMTP SIZE: Allows clients to declare email size, enabling servers to decide whether to accept it

POP3: Post Office Protocol version 3

Basic Principles and Functions

POP3 (Post Office Protocol version 3) is a standard protocol for retrieving emails from a remote server, defined in RFC 1939. POP3's main functions include:

Allowing users to download emails from the mail server to local devices
Supporting the option to retain or delete email copies on the server after downloading
Providing basic user authentication mechanisms POP3 uses TCP port 110 for plaintext communication or port 995 for SSL/TLS encrypted communication (POP3S).

Working Modes

The POP3 protocol typically operates in one of two modes:

Download and Delete Mode: Emails are deleted from the server after being downloaded to the client, suitable for single-device access scenarios
Download and Keep Mode: Copies of emails are retained on the server after downloading, allowing access from multiple devices but potentially causing storage space issues

POP3 Interaction Flow

A typical POP3 session includes the following phases:

Connection Establishment: The client connects to the POP3 server (port 110 or 995)
Authentication Phase: Using USER/PASS commands to provide username and password
Transaction Phase: Using LIST to view available emails, RETR to retrieve specified emails, DELE to mark emails for deletion, etc.
Update Phase: The client sends the QUIT command, and the server executes the marked deletion operations

Limitations of POP3

While POP3 is simple to use, it has some notable limitations:

It doesn't support partial email downloads; entire emails must be downloaded
It doesn't preserve email status information (such as read/unread) on the server
It doesn't support server-side email search and filtering
It provides a poor synchronization experience in multi-device scenarios

IMAP: Internet Message Access Protocol

Basic Principles and Functions

IMAP (Internet Message Access Protocol) is a protocol that allows email clients to access emails on a remote server. The current latest version is IMAP4rev1 (RFC 3501). Compared to POP3, IMAP offers richer functionality:

Support for managing emails on the server, including creating, deleting, and renaming email folders
Allowing email clients to download only email headers or partial content, saving bandwidth
Preserving email status information (such as read/unread, replied, flagged, etc.)
Supporting simultaneous access to multiple mailboxes
Providing server-side search capabilities IMAP uses TCP port 143 for plaintext communication or port 993 for SSL/TLS encrypted communication (IMAPS).

Advantages of IMAP

The main advantages of the IMAP protocol include:

Multi-device Synchronization: All devices can see consistent email status and organizational structure
Bandwidth Conservation: Can download only email headers or specified parts, suitable for mobile devices
Offline Operation Support: Supports organizing and managing emails while offline, synchronizing with the server upon reconnection
Server-side Search: Executes search queries on the server, eliminating the need to download all emails

IMAP Interaction Flow

An IMAP session typically includes the following phases:

Connection Establishment: The client connects to the IMAP server
Authentication: The user credentials are provided for authentication
Mailbox Selection: A mailbox is selected for operations
Command Interaction: Various operations are executed, such as FETCH (retrieve emails), STORE (modify email flags), etc.
Session Closure: The LOGOUT command is sent to end the session

Modern Email Protocol Extensions and Security Standards

MIME: Multipurpose Internet Mail Extensions

MIME (Multipurpose Internet Mail Extensions) addresses the limitations of early SMTP, which could only transmit ASCII text, allowing:

Inclusion of non-ASCII character text in emails
Addition of various attachment formats (images, audio, video, etc.)
Multiple content parts within a single email MIME defines content types and encoding methods through email header fields such as Content-Type and Content-Transfer-Encoding.

Security and Authentication Extensions

To enhance email security and prevent abuse, various security extensions have been introduced:

SPF (Sender Policy Framework): Verifies whether an email's sending server is authorized to send mail on behalf of a specific domain
DKIM (DomainKeys Identified Mail): Uses cryptographic signatures to verify the integrity of email content and the authenticity of the sender's domain
DMARC (Domain-based Message Authentication, Reporting, and Conformance): Integrates SPF and DKIM, providing reporting mechanisms
S/MIME and PGP/MIME: Provide end-to-end email encryption and digital signature capabilities
OAuth 2.0 Integration: Modern email services use OAuth for authentication, avoiding the security risks of traditional password authentication

Emerging Standards

Email protocols continue to evolve:

JMAP (JSON Meta Application Protocol): A modern JSON-based alternative designed to replace IMAP and SMTP submission
AMP for Email: Allows embedding dynamic interactive elements in emails
MTA-STS (Mail Transfer Agent Strict Transport Security): Enhances email transport security, preventing downgrade attacks

Protocol Selection Guidelines

POP3 vs. IMAP: How to Choose

Best Practices for Corporate Environments

For corporate environments, the following configurations are recommended:

Use SMTP with TLS/SSL (SMTPS or STARTTLS) for sending emails
Prefer IMAP over POP3 to support multi-device collaboration
Implement SPF, DKIM, and DMARC to verify email authenticity
Consider using S/MIME or PGP for end-to-end encryption of sensitive communications
Where allowed, use emerging standards like JMAP to improve user experience

Conclusion

Email protocols form the foundation of internet communication, and understanding their working principles and differences helps users and administrators make better configuration decisions. While basic protocols like SMTP, POP3, and IMAP have existed for decades, they continue to evolve through various extensions to meet modern communication's security and functionality requirements. As internet technology advances, we may see more innovative email protocols and standards emerge, but the core principles of email exchange will remain important. For ordinary users, understanding these protocols' basic principles helps better configure and use email services; for technical personnel, mastering these protocols is fundamental to building and maintaining reliable email systems.