Email Workers Gotchas

Critical Issues

ReadableStream Single-Use

// ❌ WRONG: Stream consumed twice
const email = await PostalMime.parse(await new Response(message.raw).arrayBuffer());
const rawText = await new Response(message.raw).text(); // EMPTY!

// ✅ CORRECT: Buffer first
const buffer = await new Response(message.raw).arrayBuffer();
const email = await PostalMime.parse(buffer);
const rawText = new TextDecoder().decode(buffer);

ctx.waitUntil() Errors Silent

// ❌ Errors dropped silently
ctx.waitUntil(fetch(webhookUrl, { method: 'POST', body: data }));

// ✅ Catch and log
ctx.waitUntil(
  fetch(webhookUrl, { method: 'POST', body: data })
    .catch(err => env.ERROR_LOG.put(`error:${Date.now()}`, err.message))
);

Security

Envelope vs Header From (Spoofing)

const envelopeFrom = message.from;               // SMTP MAIL FROM (trusted)
const headerFrom = (await PostalMime.parse(buffer)).from?.address; // (untrusted)
// Use envelope for security decisions

Input Validation

if (message.rawSize > 5_000_000) { message.setReject('Too large'); return; }
if ((message.headers.get('Subject') || '').length > 1000) {
  message.setReject('Invalid subject'); return;
}

DMARC for Replies

Replies fail silently without DMARC. Verify: dig TXT _dmarc.example.com

Parsing

Address Parsing

const email = await PostalMime.parse(buffer);
const fromAddress = email.from?.address || 'unknown';
const toAddresses = Array.isArray(email.to) ? email.to.map(t => t.address) : [email.to?.address];

Character Encoding

Let postal-mime handle decoding - email.subject, email.text, email.html are UTF-8.

API Behavior

setReject() vs throw

// setReject() for SMTP rejection
if (blockList.includes(message.from)) { message.setReject('Blocked'); return; }

// throw for worker errors
if (!env.KV) throw new Error('KV not configured');

forward() Only X-* Headers

headers.set('X-Processed-By', 'worker');  // ✅ Works
headers.set('Subject', 'Modified');        // ❌ Dropped

Reply Requires Verified Domain

// Use same domain as receiving address
const receivingDomain = message.to.split('@')[1];
await message.reply(new EmailMessage(`noreply@${receivingDomain}`, message.from, rawMime));

Performance

CPU Limit

// Skip parsing large emails
if (message.rawSize > 5_000_000) {
  await message.forward('inbox@example.com');
  return;
}

Monitor: npx wrangler tail

Limits

Common Errors