reactive-mail-server/backend/src/main/java/com/mailserver/smtp/MailRouter.java

package com.mailserver.smtp;

import com.mailserver.config.MailDomains;
import com.mailserver.delivery.OutboundQueue;
import com.mailserver.delivery.SmtpDeliveryClient;
import com.mailserver.domain.User;
import com.mailserver.repository.UserRepository;
import com.mailserver.service.DkimService;
import com.mailserver.service.LocalDelivery;
import com.mailserver.service.UserService;
import org.springframework.stereotype.Service;

import java.util.Locale;

/**
 * Where mail goes — one rule for SMTP and for the web client:
 * an existing mailbox of our domain → that mailbox; catch-all set → everything else into the catch-all mailbox
 * (nothing goes outside, existing mailboxes still get their own mail); otherwise our domain without such a mailbox →
 * "no such user", someone else's domain → the delivery queue, only for a logged-in sender (never an open relay).
 */
@Service
public class MailRouter implements SmtpBackend {

    private final MailDomains domains;
    private final UserRepository users;
    private final UserService userService;
    private final LocalDelivery localDelivery;
    private final OutboundQueue queue;
    private final DkimService dkim;
    private final SmtpDeliveryClient client;

    public MailRouter(MailDomains domains, UserRepository users, UserService userService, LocalDelivery localDelivery,
                      OutboundQueue queue, DkimService dkim, SmtpDeliveryClient client) {
        this.domains = domains;
        this.users = users;
        this.userService = userService;
        this.localDelivery = localDelivery;
        this.queue = queue;
        this.dkim = dkim;
        this.client = client;
    }

    @Override
    public boolean authenticate(String username, String password) {
        User user = users.findByEmail(username.toLowerCase(Locale.ROOT)).block();
        return user != null && user.isActive() && userService.checkPassword(user, password);
    }

    @Override
    public Route route(String recipient, String authenticatedUser) {
        if (domains.isLocal(recipient) && mailboxExists(recipient)) {
            return Route.LOCAL;
        }
        if (domains.catchAll() != null) {
            return Route.LOCAL;
        }
        if (domains.isLocal(recipient)) {
            return Route.UNKNOWN_USER;
        }
        if (authenticatedUser == null) {
            return Route.RELAY_DENIED;
        }
        return client.mode() == SmtpDeliveryClient.Mode.OFF ? Route.OUTBOUND_DISABLED : Route.OUTBOUND;
    }

    @Override
    public void accept(Envelope envelope, byte[] message) throws Exception {
        boolean trusted = envelope.authenticatedUser() != null;
        byte[] signed = null;
        for (Recipient recipient : envelope.recipients()) {
            switch (recipient.route()) {
                case LOCAL -> {
                    String mailbox = mailbox(recipient.address());
                    localDelivery.deliver(message, mailbox, envelope.remoteIp(), envelope.mailFrom(), trusted);
                }
                case OUTBOUND -> {
                    if (signed == null) {
                        signed = dkim.sign(message);
                    }
                    queue.enqueue(envelope.mailFrom(), recipient.address(), signed).block();
                }
                default -> throw new IllegalStateException("Recipient was not accepted: " + recipient);
            }
        }
    }

    /** Mailbox that stores mail to this address: its own if it exists here, otherwise the catch-all one. */
    String mailbox(String address) {
        if (domains.catchAll() == null || (domains.isLocal(address) && mailboxExists(address))) {
            return address;
        }
        return domains.catchAll();
    }

    private boolean mailboxExists(String address) {
        return Boolean.TRUE.equals(users.existsByEmail(address.toLowerCase(Locale.ROOT)).block());
    }
}