SMTP Process

Step 1: Goal Clarification

Assignment Goal

You are building a keyboard-triggered alarm simulator that:

Simulates a sensor using keyboard input.
Sends an email via SMTP to a local user’s mailbox (not the internet).
Proves delivery by reading the email in a local email client (like mail).

Real-world Analogy

This is like simulating a home alarm system that emails you when triggered — but it only uses local infrastructure (no cloud, no Gmail, no external mail servers).

What You Will Build

A Java program that:
- Listens for key presses (e.g., door opens → you hit D)
- Connects to an SMTP server (on localhost:25)
- Sends a properly-formatted email using raw SMTP commands
A Postfix mail server that:
- Accepts the email from your Java program
- Delivers it to your local user account inbox
A way to read the email locally (to prove it worked)

What You’re NOT Doing

You're not sending email to Gmail or external accounts.
You're not using any high-level mail libraries (like JavaMail).
You're not using APIs or cloud services.

Requirements Going Forward

You’ll need:

Linux OS, or WSL2 on Windows (This solution will be done with Linux OS).
Java (JDK 11+)
Postfix installed locally
Mail client (mail or mutt)

That’s Step 1: Goal clarified and scoped.

Step 2: System Setup (Linux)

We’ll set up:

A working Postfix SMTP server (local only)
A mail client to verify messages
Your Linux user account as a local recipient

Step 2.1: Install Required Packages

Install Postfix (SMTP server) and mailutils (simple mail client):

sudo apt update
sudo apt install postfix mailutils

During install, you’ll get prompted for Postfix configuration.

Step 2.2: Postfix Install Prompts

Prompt 1: "General type of mail configuration"

Choose: Local only

Prompt 2: "System mail name"

Enter: localhost

That tells Postfix: s - Only accept mail from local programs - Only deliver mail to users on your own system - Never talk to the internet

Step 2.3: Verify Postfix Config

Open the config:

sudo nano /etc/postfix/main.cf

Look for (or add if missing):

inet_interfaces = loopback-only
mydestination = localhost.localdomain, localhost

Save and exit (Ctrl + O, Enter, Ctrl + X)

Step 2.4: Restart Postfix

sudo systemctl restart postfix

Step 2.5: Test Local Delivery

Send a test email to your user (replace $USER if needed):

echo "This is a test message" | mail -s "Test Subject" $USER

Now read it:

mail

If you see the email: Postfix is working and delivering mail locally.

At this point:

Your Linux system can send/receive local emails.
Postfix is isolated (no internet delivery).
Your user has a functioning local inbox.

Step 3: Java Program to Send Raw SMTP Email

Now we write a Java program that:

Connects to your local Postfix SMTP server (localhost:25)
Sends an email using raw SMTP commands
Triggers on a key press (simulating a sensor)

Step 3.1: Basic Java SMTP Client Skeleton

Here's the full code, no libraries — just Socket, PrintWriter, and BufferedReader.

import java.io.*;
import java.net.Socket;
import java.util.Scanner;

public class AlarmSMTP {

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.println("Alarm system armed. Press any key to simulate sensor trigger...");

        scanner.nextLine(); // Wait for key press

        try (Socket socket = new Socket("localhost", 25);
             PrintWriter out = new PrintWriter(socket.getOutputStream(), true);
             BufferedReader in = new BufferedReader(new InputStreamReader(socket.getInputStream()))) {

            // Read server banner
            System.out.println("SERVER: " + in.readLine());

            // Send HELO
            out.println("HELO localhost");
            System.out.println("SERVER: " + in.readLine());

            // MAIL FROM
            out.println("MAIL FROM:<alarm@localhost>");
            System.out.println("SERVER: " + in.readLine());

            // RCPT TO (your Linux username)
            String recipient = System.getProperty("user.name") + "@localhost";
            out.println("RCPT TO:<" + recipient + ">");
            System.out.println("SERVER: " + in.readLine());

            // DATA
            out.println("DATA");
            System.out.println("SERVER: " + in.readLine());

            // Email body (ends with single period on a line)
            out.println("Subject: Alarm Triggered");
            out.println();
            out.println("Sensor was triggered at: " + java.time.LocalDateTime.now());
            out.println(".");
            System.out.println("SERVER: " + in.readLine());

            // QUIT
            out.println("QUIT");
            System.out.println("SERVER: " + in.readLine());

            System.out.println("Alarm email sent to " + recipient);

        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

Step 3.2: Compile and Run

javac AlarmSMTP.java
java AlarmSMTP

It will wait for a keypress.
Then it connects to Postfix, sends email manually via SMTP commands.
You’ll see each server response (handshake, mail accepted, etc.).
Then it exits cleanly.

Step 3.3: Check the Mail

Open the local inbox:

mail

You should see:

A subject line: Alarm Triggered
Body: includes timestamp from the program

This completes Step 3: Your Java SMTP client works, talks to Postfix, and delivers a local message.

Step 4: Make Sure You Can Read the Local Email

You've now:

Installed Postfix
Set up your Java program to send email to a local user (like you@localhost)
Sent an email via raw SMTP

Now you need to verify the message was actually delivered and can be read locally.

Step 4.1: Use the `mail` Command

The Postfix server delivers messages to the local user's mailbox, usually in /var/mail/<username>. On most Linux distros, you can read it using the mail CLI tool:

mail

If this is your first time using it, you’ll see a numbered list of messages like this:

Heirloom Mail version 12.5.  Type ? for help.
"/var/mail/yourusername": 1 message
> 1  From alarm@localhost  Fri Apr 12 12:30:22 2025
   Subject: Alarm Triggered
   ...

Press Enter to read a message.
Use q to quit.
Use d to delete a message.
Use h to list headers again.

If mail is not installed, run:

sudo apt install mailutils

Or on RHEL-based:

sudo dnf install mailx

Step 4.2: Check Mail File Directly (Optional)

If mail isn't available or you want to confirm what's stored, you can read your mail file directly:

cat /var/mail/$(whoami)

This shows the raw contents of your inbox.

Confirm Delivery

If everything worked, you should see:

An email From: alarm@localhost
A Subject: Alarm Triggered
A Body: that contains the message your Java program sent

This confirms that:

Your SMTP client works
Postfix accepted and routed the message
The mail landed in your local user’s inbox

That’s Step 4 done.

Step 5: making this program loop or respond to repeated input like a real alarm system.

Step 5: Simulate the Alarm Trigger via Keyboard Input

Now that you can send mail and verify delivery, the next step is to simulate the alarm being triggered. Per your original scenario, this is done by pressing specific keys that act like sensors (e.g., door open, motion detected).

Goal of Step 5:

Make your Java program listen for specific keyboard input, and send an email whenever a certain key is pressed.

Implementation Plan:

We'll update the Java program to:

Loop forever, waiting for key input.
On a specific key press (e.g., 'm' for motion or 'd' for door), it sends an alert email.
Maybe allow 'q' to quit the program cleanly.

Basic Java Loop for Keyboard Input

Here’s a minimal example of that:

import java.io.*;

public class AlarmTrigger {
    public static void main(String[] args) throws Exception {
        BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));

        System.out.println("Alarm system active. Press 'm' for motion, 'd' for door, 'q' to quit.");

        while (true) {
            String input = reader.readLine();

            if (input == null) continue;

            input = input.trim().toLowerCase();

            if (input.equals("q")) {
                System.out.println("Exiting alarm system.");
                break;
            }

            if (input.equals("m")) {
                sendAlarm("Motion detected!");
            } else if (input.equals("d")) {
                sendAlarm("Door opened!");
            } else {
                System.out.println("Unknown command. Use 'm', 'd', or 'q'.");
            }
        }
    }

    private static void sendAlarm(String message) {
        try {
            String recipient = System.getProperty("user.name") + "@localhost";
            Socket socket = new Socket("localhost", 25);
            BufferedReader in = new BufferedReader(new InputStreamReader(socket.getInputStream()));
            PrintWriter out = new PrintWriter(socket.getOutputStream(), true);

            // Read server greeting
            System.out.println("SERVER: " + in.readLine());

            out.println("HELO localhost");
            System.out.println("SERVER: " + in.readLine());

            out.println("MAIL FROM:<alarm@localhost>");
            System.out.println("SERVER: " + in.readLine());

            out.println("RCPT TO:<" + recipient + ">");
            System.out.println("SERVER: " + in.readLine());

            out.println("DATA");
            System.out.println("SERVER: " + in.readLine());

            out.println("Subject: Alarm Triggered");
            out.println();
            out.println("Alert: " + message);
            out.println(".");
            System.out.println("SERVER: " + in.readLine());

            out.println("QUIT");
            System.out.println("SERVER: " + in.readLine());

            socket.close();
        } catch (IOException e) {
            System.err.println("Failed to send email: " + e.getMessage());
        }
    }
}

How to Test It

Run the program from terminal:

java AlarmTrigger

Type:
m → triggers "Motion detected!" email
d → triggers "Door opened!" email
q → quits the program
Use mail to check your inbox and verify each alert arrived.