Cloudflare Email Worker that parses flags in the local-part of the email address (e.g. somerandomtext-a@example.com), even if the receipient is in the BCC. Corresponding blog post: https://daanmiddendorp.com/tech/2023/06/28/parsing-bcc-recipient-with-cloudflare-email-workers

cloudflare-email-worker-recipient-flag-parser.js

/**
 * Cloudflare Email Worker that parses flags (e.g. -a) and forwards accordingly.
 * 
 * Example: 
 * somerandomtext-a@example.com would forward to Alice.
 * somerandomtext-b@example.com would forward to Bob.
 */
async function email(message, env, ctx) {
  let messageSource = await fetchMessageSource(message.raw);
  let blacklist = await fetchBlacklist("https://example.com/blacklist.txt");
  let recipient = parseRecipient(messageSource);
  
  if (blacklist.indexOf(recipient) > -1) {
    return message.setReject("Address is blocked");
  }
  
  if (/\b.+-a@example.com\b/gi.test(recipient)) {
    await message.forward("alice@company.com");
  } else if (/\b.+-b@example.com\b/gi.test(recipient)) {
    await message.forward("bob@company.com");
  } else {
    await message.forward("catchall@company.com");
  }
}

/**
 * Fetch readable stream, so that we can convert the raw message into a string.
 * @param ReadableStream message
 * @return string messageSource
 */
async function fetchMessageSource(message) {
  const rawMessage = new Response(message);
  const arrayBuffer = await rawMessage.arrayBuffer();
  const messageSource = String.fromCharCode.apply(null, new Uint8Array(arrayBuffer));
  return messageSource;
}

/**
 * Download text file and split on newlines.
 * @param string url location of the blacklist
 * @return string[] blacklist
 */
async function fetchBlacklist(url) {
  const response = await fetch(url, {});
  const blacklist = (await response.text()).split("\n");
  return blacklist;
}

/**
 * Parse receipient from Received header (even if BCC'ed)
 * Example header:
 * 
 * Received: by mail-vk1-f194.google.com with SMTP id 71fab90a1359d-47167a4ce3cso2249208e0c.2
 * for <iets2-d@example.com>; Wed, 28 Jun 2023 04:30:15 -0700 (PDT)
 * 
 * @param string messageSource
 * @return string recipient email address
 */
function parseRecipient(messageSource) {
  return messageSource.match(/\b(?<=for <).+@example.com(?=>;)\b/gi)[0];
}

export default {
  email
}