📥 GMailReceiver Service

GMailReceiver is a Spring Boot application that displays emails in a user-friendly inbox UI and integrates seamlessly with GMailSender to send emails. It allows users to view, compose, and manage emails with ease. The service provides a clean and responsive interface for email operations.

---

🛠️ Technology Stack

• Java 17+
• Spring Boot 3.x
• Spring Web, Spring MVC
• Thymeleaf (UI templates)
• RestTemplate (for calling GMailSender service)
• Bootstrap 5 (frontend styling)
• Maven

---

🔹 Features

• Display received emails in Inbox, Starred, Sent, Spam, and Bin folders.
• Compose and send emails via GMailSender REST API.
• Show success/error messages after sending emails.
• Back to Inbox button after sending emails.
• Responsive UI with table and modal forms for email operations.

---

📸 Screenshots

Compose Mail

⚙️ Configuration

Update src/main/resources/application.properties with your settings:

spring.application.name=GEmailReceiver

# Gmail IMAP settings (for reading emails)
spring.mail.host=imap.gmail.com
spring.mail.port=993
spring.mail.username=your-email@gmail.com
spring.mail.password=your-app-password

# GMailSender service URL
gmailsender.url=http://localhost:8081

📧 How Email Works Internally — SMTP, IMAP & POP3 Explained

Emails may look simple on the surface, but under the hood, there’s a robust system of protocols that move messages reliably across the internet.
This document explains how sending and receiving emails actually works — from the sender’s outbox to the receiver’s inbox.

---

🧭 Overview

When you send or receive an email, several servers and protocols come into play:

• SMTP (Simple Mail Transfer Protocol) → handles sending emails.
• IMAP (Internet Message Access Protocol) → handles retrieving emails (keeps mail synced across devices).
• POP3 (Post Office Protocol v3) → also retrieves emails, but downloads and usually removes them from the server.

These protocols are layered on top of TCP/IP, and they use well-defined ports and security standards (like SSL/TLS).

---

🚀 Sending an Email (SMTP)

SMTP is the workhorse for sending emails.
It’s used by your mail client (like Gmail, Outlook, or Thunderbird) to deliver messages to the mail server — and by mail servers themselves to relay messages between domains.

SMTP Flow:

1. User writes an email in a client (MUA — Mail User Agent).
2. The MUA connects to an SMTP server (MSA — Mail Submission Agent) using:
   • Port 587 (with STARTTLS) or 465 (with SSL/TLS).
3. The MSA validates the user and forwards the email to the Mail Transfer Agent (MTA).
4. The MTA looks up the recipient’s domain via DNS MX (Mail Exchange) records.
5. The email is routed across the internet, possibly hopping through multiple MTAs.
6. Finally, the recipient’s Mail Delivery Agent (MDA) stores the email in their mailbox on the destination mail server.

In short:

Sender (MUA) → MSA → MTA → MDA → Recipient’s Mailbox

---

📥 Receiving Emails (IMAP vs POP3)

Once the email reaches the recipient’s mail server, the user can fetch it using either IMAP or POP3.

🔄 IMAP (Internet Message Access Protocol)

IMAP keeps your emails on the server and synchronizes changes (read/unread, folders, flags) across multiple devices.

• Default ports:
  • 143 (IMAP)
  • 993 (IMAPS - IMAP over SSL)
• Best for multi-device users — mobile, laptop, webmail, etc.

Flow:

Mail Server → IMAP Server → Client (syncs messages)

When you open an email on your phone, it’s not downloaded — you’re viewing it remotely.
Delete or move it, and the change reflects everywhere.

---

⬇️ POP3 (Post Office Protocol v3)

POP3 is older and simpler — it downloads emails from the server and (by default) removes them afterward.

• Default ports:
  • 110 (POP3)
  • 995 (POP3S - over SSL)
• Great for single-device setups, less ideal for modern multi-device use.

Flow:

Mail Server → POP3 Server → Client (downloads and optionally deletes)

Some clients can be configured to “leave a copy on the server,” but that’s a workaround — IMAP handles this natively.

---

🔐 Security and Authentication

Modern email systems always wrap these protocols in encryption:

Protocol	Default Port	Secure Port (SSL/TLS)
SMTP	25 / 587	465
IMAP	143	993
POP3	110	995

Authentication uses either:

• LOGIN / PLAIN (basic, often over TLS)
• OAuth 2.0 (modern, token-based — used by Gmail, Outlook, etc.)

---

📡 Real-World Example

Let’s say Alice sends an email to Bob:

1. Alice’s mail client connects to smtp.gmail.com:587.
2. Gmail’s MTA looks up Bob’s domain’s MX record — say mx.outlook.com.
3. Gmail’s server sends the message to Outlook’s mail server.
4. Outlook stores it in Bob’s mailbox.
5. Bob’s phone connects to imap.outlook.com:993 and retrieves/syncs the email.

---

“Email’s been around since the ‘70s — the tech is old, but it’s rock-solid once you understand the moving parts.”

---

📜 License

This project is licensed under the MIT License.