reactive-mail-server/backend/src/main/java/com/mailserver/service/LocalDelivery.java

package com.mailserver.service;

import com.mailserver.delivery.DeliveryWorker;
import com.mailserver.delivery.OutboundMessage;
import jakarta.mail.BodyPart;
import jakarta.mail.MessagingException;
import jakarta.mail.Multipart;
import jakarta.mail.Part;
import jakarta.mail.Session;
import jakarta.mail.internet.InternetAddress;
import jakarta.mail.internet.MimeMessage;
import jakarta.mail.internet.MimeUtility;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.beans.factory.annotation.Value;
import org.springframework.stereotype.Service;

import java.io.ByteArrayInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.nio.charset.StandardCharsets;
import java.util.ArrayList;
import java.util.List;
import java.util.Properties;

/**
 * Stores a message in one of our mailboxes. Mail from outside is checked first — DKIM (signature),
 * SPF (the connecting IP against the envelope sender's domain), DMARC (policy of the From domain);
 * a failure marks it as spam. Mail from our own logged-in users is trusted.
 */
@Service
public class LocalDelivery implements DeliveryWorker.BounceSender {

    private static final Logger log = LoggerFactory.getLogger(LocalDelivery.class);

    private final MailService mailService;
    private final DkimService dkimService;
    private final SpfService spfService;
    private final DmarcService dmarcService;
    private final String hostname;
    private final MessageSources sources;

    public LocalDelivery(MailService mailService, DkimService dkimService, SpfService spfService,
                         DmarcService dmarcService, @Value("${mailserver.smtp.hostname:localhost}") String hostname,
                         MessageSources sources) {
        this.sources = sources;
        this.mailService = mailService;
        this.dkimService = dkimService;
        this.spfService = spfService;
        this.dmarcService = dmarcService;
        this.hostname = hostname;
    }

    /** Blocking: call from a worker thread. */
    public void deliver(byte[] raw, String mailbox, String remoteIp, String envelopeFrom, boolean trusted)
            throws MessagingException, IOException {
        MimeMessage mime = new MimeMessage(Session.getInstance(new Properties()), new ByteArrayInputStream(raw));

        String senderEmail = envelopeFrom;
        String senderName = null;
        if (mime.getFrom() != null && mime.getFrom().length > 0 && mime.getFrom()[0] instanceof InternetAddress from) {
            senderEmail = from.getAddress();
            senderName = from.getPersonal();
        }
        String subject = mime.getSubject() == null ? "" : mime.getSubject();
        Body body = body(mime);

        boolean spam = false;
        if (!trusted) {
            DkimService.Verification dkim = dkimService.verify(raw);
            String spfSender = envelopeFrom == null || envelopeFrom.isEmpty() ? senderEmail : envelopeFrom;
            SpfService.SpfResult spf = spfService.checkSpf(spfSender, remoteIp);
            boolean spfPassed = spf == SpfService.SpfResult.PASS;
            DmarcService.DmarcResult dmarc = dmarcService.checkDmarc(senderEmail, remoteIp, spfPassed, dkim.passed(), dkim.domain());
            spam = dmarcService.shouldMarkAsSpam(dmarc) || spf == SpfService.SpfResult.FAIL || (dkim.signed() && !dkim.passed());
            log.info("Incoming {} -> {} from {}: DKIM {}, SPF {}, DMARC {}{}", senderEmail, mailbox, remoteIp,
                    dkim.signed() ? (dkim.passed() ? "pass " + dkim.domain() : "fail") : "none", spf, dmarc, spam ? " → spam" : "");
        }

        com.mailserver.domain.Message stored =
                mailService.receiveMessage(senderEmail, senderName, mailbox, subject, body.text(), body.html(), spam,
                        addresses(mime, jakarta.mail.Message.RecipientType.TO),
                        addresses(mime, jakarta.mail.Message.RecipientType.CC)).block();
        if (stored != null) {
            for (FilePart file : attachments(mime)) {
                mailService.storeIncomingAttachment(stored.getId(), file.name(), file.contentType(), file.bytes());
            }
            // IMAP serves the message exactly as it arrived
            sources.put(stored.getId(), raw).block();
        }
    }

    /** "a@x, b@y" from a header, or null. A broken header is ignored, not a reason to lose the message. */
    public static String addresses(MimeMessage mime, jakarta.mail.Message.RecipientType type) {
        try {
            jakarta.mail.Address[] list = mime.getRecipients(type);
            if (list == null || list.length == 0) {
                return null;
            }
            List<String> result = new ArrayList<>();
            for (jakarta.mail.Address address : list) {
                result.add(address instanceof InternetAddress internet ? internet.getAddress() : address.toString());
            }
            String joined = String.join(", ", result);
            return joined.length() > 5000 ? joined.substring(0, 5000) : joined;
        } catch (MessagingException e) {
            return null;
        }
    }

    public record FilePart(String name, String contentType, byte[] bytes) {
    }

    /** Parts with a file name (attachments and inline images), nested multiparts walked. */
    public static List<FilePart> attachments(Part part) throws MessagingException, IOException {
        List<FilePart> files = new ArrayList<>();
        collect(part, files);
        return files;
    }

    private static void collect(Part part, List<FilePart> files) throws MessagingException, IOException {
        if (part.isMimeType("multipart/*") && part.getContent() instanceof Multipart multipart) {
            for (int i = 0; i < multipart.getCount(); i++) {
                collect(multipart.getBodyPart(i), files);
            }
            return;
        }
        String name = part.getFileName();
        if (name == null && !Part.ATTACHMENT.equalsIgnoreCase(part.getDisposition())) {
            return;
        }
        String decoded = name == null ? "file" : MimeUtility.decodeText(name);
        String type = part.getContentType() == null ? "application/octet-stream" : part.getContentType().split(";")[0].trim();
        try (InputStream in = part.getInputStream()) {
            files.add(new FilePart(decoded, type.toLowerCase(java.util.Locale.ROOT), in.readAllBytes()));
        }
    }

    /** Non-delivery report into the sender's inbox — only when the sender is our mailbox. */
    @Override
    public void bounce(OutboundMessage message, String reason) {
        String text = "Your message to " + message.getRcptTo() + " could not be delivered.\n\n"
                + "Reason: " + reason + "\n\nAttempts: " + message.getAttempts() + "\n";
        try {
            mailService.receiveMessage("mailer-daemon@" + hostname, "Mail Delivery System", message.getMailFrom(),
                    "Undelivered: message to " + message.getRcptTo(), text, false, false).block();
        } catch (IllegalArgumentException notOurs) {
            log.info("Bounce not stored: sender {} is not a local mailbox", message.getMailFrom());
        }
    }

    public record Body(String text, boolean html) {
    }

    /** HTML part when the message has one, otherwise the plain text; nested multiparts are walked. */
    public static Body body(Part part) throws MessagingException, IOException {
        String html = find(part, "text/html");
        if (html != null) {
            return new Body(html, true);
        }
        String plain = find(part, "text/plain");
        return new Body(plain == null ? "" : plain, false);
    }

    private static String find(Part part, String type) throws MessagingException, IOException {
        if (Part.ATTACHMENT.equalsIgnoreCase(part.getDisposition()) || part.getFileName() != null) {
            return null;
        }
        if (part.isMimeType(type)) {
            Object content = part.getContent();
            if (content instanceof String s) {
                return s;
            }
            if (content instanceof InputStream in) {
                try (in) {
                    return new String(in.readAllBytes(), StandardCharsets.UTF_8);
                }
            }
            return null;
        }
        if (part.isMimeType("multipart/*") && part.getContent() instanceof Multipart multipart) {
            for (int i = 0; i < multipart.getCount(); i++) {
                BodyPart child = multipart.getBodyPart(i);
                String found = find(child, type);
                if (found != null) {
                    return found;
                }
            }
        }
        return null;
    }
}