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

package com.mailserver.service;

import com.mailserver.domain.Attachment;
import com.mailserver.domain.User;
import com.mailserver.dto.MessageResponse;
import com.mailserver.dto.SendRequest;
import com.mailserver.repository.UserRepository;
import com.mailserver.smtp.MailRouter;
import com.mailserver.smtp.SmtpBackend;
import jakarta.mail.Message;
import jakarta.mail.MessagingException;
import jakarta.mail.Session;
import jakarta.mail.internet.AddressException;
import jakarta.mail.internet.InternetAddress;
import jakarta.mail.internet.MimeBodyPart;
import jakarta.mail.internet.MimeMessage;
import jakarta.mail.internet.MimeMultipart;
import jakarta.mail.internet.MimeUtility;
import org.springframework.beans.factory.annotation.Value;
import org.springframework.http.HttpStatus;
import org.springframework.stereotype.Service;
import org.springframework.web.server.ResponseStatusException;
import reactor.core.publisher.Flux;
import reactor.core.publisher.Mono;
import reactor.core.scheduler.Schedulers;

import java.io.ByteArrayOutputStream;
import java.io.File;
import java.io.IOException;
import java.util.ArrayList;
import java.util.Date;
import java.util.LinkedHashSet;
import java.util.List;
import java.util.Locale;
import java.util.Properties;
import java.util.Set;
import java.util.UUID;

/**
 * "Send" in the web client: builds the MIME message (with the draft's attachments) and routes it like an
 * authenticated SMTP submission — to every To and Cc address.
 */
@Service
public class SubmissionService {

    /** One message, at most this many addresses (To + Cc). */
    static final int MAX_RECIPIENTS = 50;

    private final UserRepository users;
    private final MailService mailService;
    private final MailRouter router;
    private final String hostname;

    public SubmissionService(UserRepository users, MailService mailService, MailRouter router,
                             @Value("${mailserver.smtp.hostname:localhost}") String hostname) {
        this.users = users;
        this.mailService = mailService;
        this.router = router;
        this.hostname = hostname;
    }

    public Mono<MessageResponse> send(String fromEmail, SendRequest request) {
        List<String> to = addresses(request.getTo());
        List<String> cc = addresses(request.getCc());
        cc.removeAll(to);
        if (to.isEmpty()) {
            return Mono.error(MailService.badRequest("No recipient"));
        }
        if (to.size() + cc.size() > MAX_RECIPIENTS) {
            return Mono.error(MailService.badRequest("At most " + MAX_RECIPIENTS + " recipients"));
        }
        Long draftId = request.getDraftId();
        return users.findByEmail(fromEmail)
                .switchIfEmpty(Mono.error(new ResponseStatusException(HttpStatus.UNAUTHORIZED)))
                .flatMap(user -> (draftId == null ? Flux.<Attachment>empty() : mailService.draftAttachments(draftId, user.getId()))
                        .collectList()
                        .flatMap(attachments -> Mono.fromCallable(() -> submit(user, to, cc, request, attachments))
                                .subscribeOn(Schedulers.boundedElastic()))
                        .flatMap(messageId -> mailService.saveSent(user, String.join(", ", to), String.join(", ", cc),
                                request.getSubject(), request.getBody(), request.isHtml(), messageId))
                        .flatMap(sent -> (draftId == null ? Mono.<Void>empty() : mailService.finishDraft(draftId, sent.getId()))
                                .then(mailService.getMessage(sent.getId(), user.getId()))));
    }

    private String submit(User user, List<String> to, List<String> cc, SendRequest request, List<Attachment> attachments)
            throws Exception {
        List<SmtpBackend.Recipient> recipients = new ArrayList<>();
        for (String address : concat(to, cc)) {
            SmtpBackend.Route route = router.route(address, user.getEmail());
            switch (route) {
                case LOCAL, OUTBOUND -> recipients.add(new SmtpBackend.Recipient(address, route));
                case UNKNOWN_USER -> throw MailService.badRequest("No such mailbox: " + address);
                case OUTBOUND_DISABLED -> throw MailService.badRequest("Sending outside is disabled on this server: " + address);
                case RELAY_DENIED -> throw new IllegalStateException("logged-in user denied relay");
            }
        }
        String messageId = UUID.randomUUID() + "@" + hostname;
        MimeMessage mime = mime(user, to, cc, request, attachments);
        mime.setHeader("Message-ID", "<" + messageId + ">");
        ByteArrayOutputStream bytes = new ByteArrayOutputStream();
        mime.writeTo(bytes);
        router.accept(new SmtpBackend.Envelope("127.0.0.1", hostname, user.getEmail(), user.getEmail(), recipients),
                bytes.toByteArray());
        return messageId;
    }

    static MimeMessage mime(User user, List<String> to, List<String> cc, SendRequest request, List<Attachment> attachments)
            throws MessagingException, IOException {
        MimeMessage mime = new MimeMessage(Session.getInstance(new Properties()));
        mime.setFrom(new InternetAddress(user.getEmail(), user.getDisplayName(), "UTF-8"));
        mime.setRecipients(Message.RecipientType.TO, InternetAddress.parse(String.join(",", to)));
        if (!cc.isEmpty()) {
            mime.setRecipients(Message.RecipientType.CC, InternetAddress.parse(String.join(",", cc)));
        }
        mime.setSubject(request.getSubject(), "UTF-8");
        String subtype = request.isHtml() ? "html" : "plain";
        if (attachments.isEmpty()) {
            mime.setText(request.getBody(), "UTF-8", subtype);
        } else {
            MimeMultipart mixed = new MimeMultipart("mixed");
            MimeBodyPart text = new MimeBodyPart();
            text.setText(request.getBody(), "UTF-8", subtype);
            mixed.addBodyPart(text);
            for (Attachment attachment : attachments) {
                MimeBodyPart file = new MimeBodyPart();
                file.attachFile(new File(attachment.getFilePath()),
                        attachment.getContentType() == null ? "application/octet-stream" : attachment.getContentType(), null);
                file.setFileName(MimeUtility.encodeText(attachment.getFilename(), "UTF-8", null));
                mixed.addBodyPart(file);
            }
            mime.setContent(mixed);
        }
        mime.setSentDate(new Date());
        mime.saveChanges();
        return mime;
    }

    /** "a@x, b@y; c@z" → lower-case addresses without repeats; a malformed one → 400. */
    static List<String> addresses(String raw) {
        Set<String> result = new LinkedHashSet<>();
        if (raw == null) {
            return new ArrayList<>();
        }
        for (String part : raw.split("[,;\\s]+")) {
            String address = part.trim().toLowerCase(Locale.ROOT);
            if (address.isEmpty()) {
                continue;
            }
            try {
                new InternetAddress(address, true).validate();
            } catch (AddressException e) {
                throw MailService.badRequest("Invalid address: " + address);
            }
            if (!address.contains("@")) {
                throw MailService.badRequest("Invalid address: " + address);
            }
            result.add(address);
        }
        return new ArrayList<>(result);
    }

    private static List<String> concat(List<String> a, List<String> b) {
        List<String> all = new ArrayList<>(a);
        all.addAll(b);
        return all;
    }

}