Mailgun bounces are hard to debug

Yes — Mailgun bounce handling can be painful because the useful details are often buried in webhook payloads, SMTP response codes, or provider-side logs.

A practical way to debug them:

1. Check the event type

   • bounced vs dropped vs complained
   • dropped often means Mailgun never attempted delivery, or rejected it early.

2. Inspect the bounce code and error

   • SMTP status codes matter a lot:
     • 4xx = temporary failure
     • 5xx = permanent failure
   • The error field usually has the best human-readable clue.

3. Look at the recipient domain’s behavior

   • Some domains rate-limit, greylist, or block based on reputation.
   • Try sending to a known good mailbox at the same domain if possible.

4. Verify SPF, DKIM, and DMARC

   • Misalignment can cause silent reputation issues or outright rejection.
   • Especially important if bounces are inconsistent across providers.

5. Check Mailgun logs

   • Search by recipient, message ID, or timestamp.
   • Compare accepted vs rejected deliveries.

6. Test with a controlled mailbox

   • Send to Gmail, Outlook, Yahoo, and a custom domain.
   • Different providers fail in different ways, which helps isolate the issue.

7. Watch for content-related blocks

   • Spammy subject lines, links, or malformed HTML can trigger rejections.
   • Try sending a minimal plain-text message.

8. Correlate with webhook retries

   • Sometimes the bounce event arrives later than the original send.
   • Make sure your system isn’t treating delayed events as duplicates or ignoring them.

If you want, I can help you build a bounce triage checklist or show you how to decode a real Mailgun bounce payload.