reactive-mail-server/backend/src/test/java/com/mailserver/delivery/SmtpDeliveryClientTest.java

package com.mailserver.delivery;

import com.icegreen.greenmail.util.GreenMail;
import com.icegreen.greenmail.util.ServerSetup;
import jakarta.mail.internet.MimeMessage;
import org.junit.jupiter.api.AfterEach;
import org.junit.jupiter.api.BeforeEach;
import org.junit.jupiter.api.Test;

import java.io.ByteArrayOutputStream;
import java.net.ServerSocket;
import java.nio.charset.StandardCharsets;
import java.util.List;

import static org.assertj.core.api.Assertions.assertThat;

class SmtpDeliveryClientTest {

    private static final String MESSAGE = "From: alice@ours.test\r\nTo: bob@theirs.test\r\nSubject: Hi\r\n"
            + "DKIM-Signature: v=1; a=rsa-sha256; d=ours.test; s=mail; h=from; bh=x; b=y\r\n\r\nbody\r\n.dot line\r\n";

    private GreenMail greenMail;

    @BeforeEach
    void start() {
        greenMail = new GreenMail(new ServerSetup(0, "127.0.0.1", ServerSetup.PROTOCOL_SMTP));
        greenMail.setUser("relay-user", "relay-pass");
        greenMail.start();
    }

    @AfterEach
    void stop() {
        greenMail.stop();
    }

    private int port() {
        return greenMail.getSmtp().getPort();
    }

    private SmtpDeliveryClient mxClient(int mxPort, List<String> hosts) {
        return new SmtpDeliveryClient(new SmtpDeliveryClient.Settings(SmtpDeliveryClient.Mode.MX, "", 0, "", "",
                SmtpDeliveryClient.Tls.NONE, "mx.ours.test", 5000, mxPort), domain -> hosts);
    }

    @Test
    void deliversToTheMxAndKeepsTheBytesUnchanged() throws Exception {
        SmtpDeliveryClient.Result result = mxClient(port(), List.of("127.0.0.1"))
                .deliver("alice@ours.test", "bob@theirs.test", MESSAGE.getBytes(StandardCharsets.UTF_8));

        assertThat(result.outcome()).isEqualTo(SmtpDeliveryClient.Outcome.DELIVERED);
        MimeMessage[] received = greenMail.getReceivedMessages();
        assertThat(received).hasSize(1);
        ByteArrayOutputStream out = new ByteArrayOutputStream();
        received[0].writeTo(out);
        // GreenMail adds its Received/Return-Path; everything we sent is there byte for byte (DKIM stays valid)
        assertThat(out.toString(StandardCharsets.UTF_8)).contains(MESSAGE.stripTrailing());
    }

    @Test
    void triesTheNextMxWhenTheFirstDoesNotAnswer() throws Exception {
        int closed;
        try (ServerSocket socket = new ServerSocket(0)) {
            closed = socket.getLocalPort();
        }
        // first "host" refuses the connection, second is GreenMail — both on 127.0.0.1, so use two clients' ports:
        SmtpDeliveryClient.Result refused = mxClient(closed, List.of("127.0.0.1"))
                .deliver("alice@ours.test", "bob@theirs.test", MESSAGE.getBytes(StandardCharsets.UTF_8));
        assertThat(refused.outcome()).isEqualTo(SmtpDeliveryClient.Outcome.TEMPORARY);

        SmtpDeliveryClient.Result fallback = mxClient(port(), List.of("unreachable.invalid", "127.0.0.1"))
                .deliver("alice@ours.test", "bob@theirs.test", MESSAGE.getBytes(StandardCharsets.UTF_8));
        assertThat(fallback.outcome()).isEqualTo(SmtpDeliveryClient.Outcome.DELIVERED);
    }

    @Test
    void domainWithoutMailServerIsAPermanentFailure() {
        SmtpDeliveryClient.Result result = mxClient(port(), List.of())
                .deliver("alice@ours.test", "bob@nowhere.test", MESSAGE.getBytes(StandardCharsets.UTF_8));
        assertThat(result.outcome()).isEqualTo(SmtpDeliveryClient.Outcome.PERMANENT);
    }

    @Test
    void dnsTroubleIsTemporary() {
        SmtpDeliveryClient client = new SmtpDeliveryClient(new SmtpDeliveryClient.Settings(SmtpDeliveryClient.Mode.MX,
                "", 0, "", "", SmtpDeliveryClient.Tls.NONE, "mx.ours.test", 5000, port()),
                domain -> { throw new SmtpDeliveryClient.TemporaryDnsException("SERVFAIL"); });
        assertThat(client.deliver("a@ours.test", "b@theirs.test", MESSAGE.getBytes()).outcome())
                .isEqualTo(SmtpDeliveryClient.Outcome.TEMPORARY);
    }

    @Test
    void sendsThroughASmarthostWithLogin() {
        SmtpDeliveryClient client = new SmtpDeliveryClient(new SmtpDeliveryClient.Settings(SmtpDeliveryClient.Mode.SMARTHOST,
                "127.0.0.1", port(), "relay-user", "relay-pass", SmtpDeliveryClient.Tls.NONE, "mx.ours.test", 5000, 25));

        assertThat(client.deliver("alice@ours.test", "bob@theirs.test", MESSAGE.getBytes()).outcome())
                .isEqualTo(SmtpDeliveryClient.Outcome.DELIVERED);
        assertThat(greenMail.getReceivedMessages()).hasSize(1);
    }

    @Test
    void offModeSendsNothing() {
        SmtpDeliveryClient client = new SmtpDeliveryClient(new SmtpDeliveryClient.Settings(SmtpDeliveryClient.Mode.OFF,
                "", 0, "", "", SmtpDeliveryClient.Tls.NONE, "mx.ours.test", 5000, 25));
        assertThat(client.deliver("a@ours.test", "b@theirs.test", MESSAGE.getBytes()).outcome())
                .isEqualTo(SmtpDeliveryClient.Outcome.PERMANENT);
        assertThat(greenMail.getReceivedMessages()).isEmpty();
    }
}