SMTPClient.java

import java.io.*;
import java.net.*;
import java.util.Date;

public class SMTPClient {

    private static final int DEFAULT_PORT = 25;
    private static final int TIMEOUT_MS = 15_000;

    private Socket socket;
    private BufferedReader reader;
    private PrintWriter writer;

    public void connect(String host, int port) throws IOException {
        socket = new Socket();
        socket.connect(new InetSocketAddress(host, port), TIMEOUT_MS);
        socket.setSoTimeout(TIMEOUT_MS);
        reader = new BufferedReader(new InputStreamReader(socket.getInputStream(), "UTF-8"));
        writer = new PrintWriter(new OutputStreamWriter(socket.getOutputStream(), "UTF-8"), true);

        requireCode(readResponse(), "220", "greeting");

        String ehlo = sendCommand("EHLO " + localHostname());
        if (!ehlo.startsWith("250")) {
            requireCode(sendCommand("HELO " + localHostname()), "250", "HELO");
        }
    }

    public void connect(String host) throws IOException {
        connect(host, DEFAULT_PORT);
    }

    public void disconnect() {
        try {
            if (socket != null && !socket.isClosed()) {
                sendCommand("QUIT");
                socket.close();
            }
        } catch (IOException ignored) {
        }
    }

    public void sendReply(String fromAddress, String toAddress,
            String originalSubject, String vacationBody) throws IOException {

        requireCode(sendCommand("MAIL FROM:<" + fromAddress + ">"), "250", "MAIL FROM");
        requireCode(sendCommand("RCPT TO:<" + toAddress + ">"), "250", "RCPT TO");
        requireCode(sendCommand("DATA"), "354", "DATA");

        StringBuilder msg = new StringBuilder();
        msg.append("From: ").append(fromAddress).append("\r\n");
        msg.append("To: ").append(toAddress).append("\r\n");
        msg.append("Subject: Re: ").append(originalSubject).append("\r\n");
        msg.append("Date: ").append(new Date()).append("\r\n");
        msg.append("Auto-Submitted: auto-replied\r\n");
        msg.append("Precedence: bulk\r\n");
        msg.append("X-Auto-Reply-To: ").append(toAddress).append("\r\n");
        msg.append("\r\n");
        msg.append(vacationBody).append("\r\n");
        for (String line : msg.toString().split("\r\n", -1)) {
            writer.print((line.startsWith(".") ? "." + line : line) + "\r\n");
        }
        writer.print(".\r\n"); // DATA terminator
        writer.flush();

        requireCode(readResponse(), "250", "end of DATA");
    }

    private String readResponse() throws IOException {
        StringBuilder sb = new StringBuilder();
        String line;
        while ((line = reader.readLine()) != null) {
            sb.append(line);
            if (line.length() < 4 || line.charAt(3) == ' ')
                break;
            sb.append("\n");
        }
        return sb.toString();
    }

    private String sendCommand(String cmd) throws IOException {
        writer.print(cmd + "\r\n");
        writer.flush();
        return readResponse();
    }

    private void requireCode(String resp, String code, String cmd) throws IOException {
        if (!resp.startsWith(code))
            throw new IOException("SMTP '" + cmd + "' expected " + code + ", got: " + resp);
    }

    private String localHostname() {
        try {
            return InetAddress.getLocalHost().getHostName();
        } catch (UnknownHostException e) {
            return "localhost";
        }
    }
}