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.MailLists;
import com.mailserver.service.MailboxAccess;
import com.mailserver.service.UserService;
import org.springframework.stereotype.Service;

import java.util.LinkedHashSet;
import java.util.Locale;
import java.util.Set;

/**
 * Where mail goes — one rule for SMTP and for the web client:
 * <ul>
 *   <li>a mailbox of ours → that mailbox; {@code bob+prod@} → bob's folder "prod" (created on first mail);</li>
 *   <li>a mailing list of ours ({@code alerts-prod@}) → a copy to every member, into the list's folder
 *       (or the "+tag" folder); a closed list takes mail only from logged-in senders;</li>
 *   <li>a TRAP sender (staging@) and an address that is not ours → nothing leaves: one copy into the sender's INBOX
 *       (our own mailboxes and lists still get their mail);</li>
 *   <li>catch-all set → everything else into the catch-all mailbox; otherwise our domain → "no such user",
 *       someone else's domain → the delivery queue, only for a logged-in sender (never an open relay).</li>
 * </ul>
 */
@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;
    private final MailLists lists;
    private final MailboxAccess access;

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

    @Override
    public String authenticate(String username, String password) {
        User mailbox = access.login(username, password);
        return mailbox == null ? null : mailbox.getEmail();
    }

    @Override
    public Route route(String recipient, String authenticatedUser) {
        if (domains.isLocal(recipient)) {
            String base = MailDomains.baseOf(recipient);
            if (mailboxExists(recipient) || mailboxExists(base)) {
                return Route.LOCAL;
            }
            MailLists.MailList list = lists.find(base).block();
            if (list != null) {
                return list.openToOutside() || authenticatedUser != null ? Route.LOCAL : Route.LIST_CLOSED;
            }
        }
        if (isTrap(authenticatedUser)) {
            return Route.TRAP;
        }
        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;
        // one copy per mailbox + folder, even when an address and a list both lead there
        Set<Target> targets = new LinkedHashSet<>();
        byte[] signed = null;
        for (Recipient recipient : envelope.recipients()) {
            switch (recipient.route()) {
                case LOCAL -> targets.addAll(targets(recipient.address()));
                case TRAP -> targets.add(new Target(envelope.authenticatedUser().toLowerCase(Locale.ROOT), null));
                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);
            }
        }
        for (Target target : targets) {
            localDelivery.deliver(message, target.mailbox(), target.folder(), envelope.remoteIp(), envelope.mailFrom(), trusted);
        }
    }

    /** Mailbox + folder (null — INBOX) that store mail to a local address. */
    record Target(String mailbox, String folder) {
    }

    Set<Target> targets(String address) {
        Set<Target> result = new LinkedHashSet<>();
        String lower = address.toLowerCase(Locale.ROOT);
        if (domains.isLocal(lower)) {
            if (mailboxExists(lower)) {
                result.add(new Target(lower, null));
                return result;
            }
            String base = MailDomains.baseOf(lower);
            String tag = MailDomains.tagOf(address);
            if (mailboxExists(base)) {
                result.add(new Target(base, tag));
                return result;
            }
            MailLists.MailList list = lists.find(base).block();
            if (list != null) {
                for (String member : list.members()) {
                    result.add(new Target(member, tag != null ? tag : list.folder()));
                }
                return result;
            }
        }
        if (domains.catchAll() != null) {
            result.add(new Target(domains.catchAll(), null));
        }
        return result;
    }

    /** Mailbox that stores mail to this address: its own (without "+tag") if it exists here, otherwise the catch-all one. */
    String mailbox(String address) {
        Set<Target> found = targets(address);
        return found.isEmpty() ? address : found.iterator().next().mailbox();
    }

    /** The logged-in sender is a trap mailbox. */
    boolean isTrap(String authenticatedUser) {
        if (authenticatedUser == null) {
            return false;
        }
        User user = users.findByEmail(authenticatedUser.toLowerCase(Locale.ROOT)).block();
        return user != null && user.isTrap();
    }

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