📧 SMTP Protocol Simulator

What is SMTP?

SMTP (Simple Mail Transfer Protocol) is an application layer protocol used for sending and routing emails across the Internet. It operates over TCP, typically on port 25, using a client–server model. Commands like HELO, MAIL FROM, RCPT TO, DATA, and QUIT control the communication. SMTP ensures reliable delivery between mail servers and supports authentication and encryption through extensions.

SMTP Commands

Commands Used in This Simulator:

• HELO/EHLO: Client introduces itself to the server and lists supported features
• MAIL FROM: Defines the sender's email address
• RCPT TO: Defines the recipient's email address
• DATA: Starts sending the message content and ends with a dot
• QUIT: Ends the SMTP session

Additional SMTP Commands:

• RSET: Clears the current mail transaction but keeps the connection
• VRFY: Checks if a user or mailbox exists
• EXPN: Shows all members of a mailing list
• NOOP: Tests or keeps the connection alive
• STARTTLS: Switches to a secure encrypted connection
• AUTH: Authenticates the client to the mail server

Store-and-Forward Mechanism

This simulator demonstrates the store-and-forward mechanism used in email systems:

• Receiver Online: Email is delivered immediately to the recipient's mailbox
• Receiver Offline: Email is stored in the server queue and delivered when receiver comes online
• Queue Management: Server maintains a queue of pending emails
• Automatic Delivery: System checks receiver status periodically and delivers queued emails

How This Project Was Built

Technologies Used:

• HTML for structure and semantic markup
• CSS for styling with modern gradients and animations
• JavaScript for SMTP protocol simulation logic

Implementation Steps:

1. Designed the user interface with input forms and visualization area
2. Implemented SMTP command sequence (HELO → MAIL FROM → RCPT TO → DATA → QUIT)
3. Added network simulation with configurable delays and packet loss
4. Created packet animation system using CSS animations
5. Built real-time logging system with color-coded entries
6. Implemented statistics tracking and error handling
7. Added receiver status management and email queuing system
8. Implemented store-and-forward mechanism for offline receivers

Network Simulation Features

• Server Delay: Simulates processing time at SMTP server
• Network Delay: Simulates packet transmission time
• Packet Loss: Random packet drops with automatic retransmission
• Receiver Status: Toggle receiver online/offline to test queuing
• Email Queue: Emails stored when receiver is offline

📺 Video Tutorial

Watch this video to learn more about how SMTP protocol works

References

• RFC 5321 - Simple Mail Transfer Protocol
• RFC 2821 - Simple Mail Transfer Protocol (Previous Standard)
• Computer Networks by Andrew S. Tanenbaum
• Data Communications and Networking by Forouzan

Step 1: Enter Email Details

Fill in the following fields in the left panel:

• Sender Email: Enter the sender's email address (e.g., alice@example.com)
• Recipient Email: Enter the recipient's email address (e.g., bob@example.com)
• Subject: Type the email subject line
• Message Body: Enter your email message content
• Attachment: (Optional) Enter attachment name like "document.pdf"

Step 2: Configure Network Conditions

Adjust network simulation parameters:

• Server Delay: Set processing time (0-5 seconds). Higher values simulate slower servers.
• Network Delay: Set packet transmission time (0-2000 ms). Higher values simulate slower networks.
• Packet Loss Rate: Set probability of packet loss (0-90%). Higher values simulate unreliable networks.

Step 3: Control Receiver Status (NEW)

Test the store-and-forward mechanism:

• Toggle Receiver Button: Click to switch receiver between online (🟢) and offline (🔴) states
• Receiver Online: Emails are delivered immediately
• Receiver Offline: Emails are queued on the server and delivered when receiver comes online
• Queued Emails Counter: Shows number of emails waiting for delivery

Step 4: Send Email

Click the "📤 Send Email" button to start the simulation.

Understanding the Visualization

• Network Diagram: Shows three nodes (Client, SMTP Server, Recipient Server)
• Animated Packets: Blue circles moving between nodes represent data packets
• Active Nodes: Highlighted nodes indicate current processing location
• Recipient Node Opacity: Dimmed when receiver is offline, full brightness when online
• Progress Bar: Shows overall transmission progress (0-100%)
• Status Badge: Displays current status (Ready, Processing, Success, Failed)

Reading the SMTP Log

The log console shows real-time SMTP commands and responses:

• Cyan Text (→ CLIENT): Commands sent by client
• Orange Text (← SERVER): Responses from server
• Yellow Background: Warnings (packet loss, retransmission, receiver offline)
• Red Background: Errors (invalid recipient, failures)
• Green Background: Success messages (delivery complete, receiver online)

Understanding Statistics

• Total Packets: Number of packets sent during transmission
• Packets Lost: Number of packets that failed to reach destination
• Retransmissions: Number of times packets were resent after loss
• Queued Emails: Number of emails waiting for offline receiver to come online

Testing Scenarios

Error Scenarios:

• Enter "invalid@invalid.com" as recipient to trigger validation error
• Set packet loss to 50-70% to see multiple retransmissions
• Use high server delay (3-5 sec) to simulate slow servers

Store-and-Forward Testing:

• Set receiver to offline (🔴) before sending email
• Watch email get queued on server
• Toggle receiver to online (🟢) during transmission
• Observe automatic delivery from queue

Execution Flow

1. Client initiates connection with HELO command
2. Client specifies sender with MAIL FROM command
3. Client specifies recipient with RCPT TO command
4. Server validates recipient address
5. Client sends email content with DATA command
6. Server accepts and stores message in queue
7. Server checks recipient server status
8. If receiver offline: Email queued, waiting for receiver to come online
9. If receiver online: Message forwarded immediately for delivery
10. Client closes connection with QUIT command
11. Delivery complete - email delivered to recipient's mailbox