Email Workers Gotchas

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