This email system project is an excellent advanced Java project that demonstrates enterprise-level communication infrastructure. Let me create a comprehensive implementation that showcases the core components of an email system.I've created a comprehensive email system implementation that demonstrates the core concepts you outlined. Here's what the system includes:

Key Features

1. Core Email Infrastructure:

• Email Class: Represents email messages with metadata (sender, recipient, subject, body, timestamp, status)
• Message ID Generation: Unique identifier for each email using MD5 hashing
• Email Status Tracking: DRAFT, SENT, DELIVERED, READ, FAILED

2. SMTP Server Implementation:

• Multi-threaded Server: Handles multiple concurrent connections
• SMTP Protocol Support: Implements core SMTP commands (HELO, MAIL FROM, RCPT TO, DATA, QUIT)
• Email Processing: Parses incoming emails and routes them to recipients
• Mailbox Management: Stores emails for each user

3. POP3 Server Implementation:

• Email Retrieval: Allows clients to download emails
• Authentication: Basic user authentication system
• POP3 Commands: STAT, LIST, RETR, USER, PASS, QUIT
• Message Status Updates: Marks emails as read when retrieved

4. Email Client GUI:

• Inbox Management: View received emails in a table format
• Email Composition: Send new emails with validation
• Real-time Updates: Auto-refresh inbox every 5 seconds
• Email Display: Read full email content with proper formatting

5. Advanced Features:

• Thread Pool Management: Efficient handling of concurrent connections
• Email Validation: Regex-based email address validation
• Error Handling: Comprehensive exception handling throughout
• Sample Data: Pre-loaded sample emails for demonstration

Technical Implementation

Protocols Used:

• SMTP (Port 2525): For sending emails
• POP3 (Port 1100): For retrieving emails
• HTTP compatibility: Can be extended to use port 80

Security Features:

• Connection Management: Proper socket handling and cleanup
• Input Validation: Email format validation and sanitization
• Thread Safety: Concurrent access protection using synchronized methods

Architecture Benefits:

• Modular Design: Separate classes for different responsibilities
• Scalable: Thread pool can handle multiple simultaneous connections
• Extensible: Easy to add new features like attachments, encryption, etc.

Usage Instructions

1. Run the Application: Execute the main method to start both servers
2. Email Clients: Two client windows will open for demo users
3. Send Emails: Use the "Compose" tab to send emails between users
4. View Emails: Check the "Inbox" tab to see received messages
5. Real-time Updates: Inbox refreshes automatically

This implementation showcases enterprise-level email system concepts including proper protocol handling, concurrent processing, and a user-friendly interface. The system can be extended with additional features like SSL/TLS encryption, attachment support, spam filtering, and database persistence.

The project demonstrates advanced Java concepts including networking, threading, GUI development, and protocol implementation - making it an excellent learning project for understanding how email systems work at a fundamental level.