LMTP server

import { SMTPServer } from "bun-smtp";
import type { DataStream, SMTPSession, SMTPError } from "bun-smtp";

const validRecipients = new Set([
  "alice@localhost",
  "bob@localhost",
  "charlie@localhost",
]);

const server = new SMTPServer({
  lmtp: true, // Enable LMTP mode
  authOptional: true,
  
  onRcptTo(address, session, callback) {
    // Validate each recipient (but don't reject yet)
    if (!validRecipients.has(address.address)) {
      console.log(`Unknown recipient: ${address.address}`);
    }
    
    // Accept all recipients during RCPT TO phase
    callback(null);
  },
  
  onData(stream: DataStream, session: SMTPSession, callback) {
    async function deliverToMailboxes() {
      const chunks: Uint8Array[] = [];
      
      // Collect the message
      for await (const chunk of stream) {
        chunks.push(chunk);
      }
      
      const message = Buffer.concat(chunks);
      const responses: Array<string | SMTPError> = [];
      
      // Attempt delivery to each recipient
      for (const recipient of session.envelope.rcptTo) {
        const address = recipient.address;
        
        if (!validRecipients.has(address)) {
          // Create an error for this specific recipient
          const err = new Error(`No such user: ${address}`) as SMTPError;
          err.responseCode = 550;
          responses.push(err);
          console.log(`✗ Delivery failed for ${address}`);
        } else {
          // Deliver to mailbox
          try {
            const maildir = `./mail/${address.replace("@", "-")}`;
            await Bun.write(`${maildir}/${Date.now()}.eml`, message);
            responses.push(`Delivered to ${address}`);
            console.log(`✓ Delivered to ${address}`);
          } catch (error) {
            const err = new Error(`Mailbox error: ${error.message}`) as SMTPError;
            err.responseCode = 452;
            responses.push(err);
            console.log(`✗ Mailbox error for ${address}`);
          }
        }
      }
      
      // Return per-recipient responses
      callback(null, responses);
    }
    
    deliverToMailboxes().catch(callback);
  },
});

await server.listen(2424);
console.log("LMTP server listening on port 2424");