When transactional emails fail to arrive, debugging requires a systematic approach. The problem could be anywhere in the pipeline: your application, the email provider, the receiving server, or the recipient's inbox.
Start with your application logs
Before looking elsewhere, confirm what your application actually did:
- Did the email send request return success or an error?
- What was the exact error message from your email provider?
- Was the recipient address formatted correctly?
- Did the message ID get generated and logged?
- Was the email queued or sent immediately?
If your application logs show an error, that is your starting point. If they show success, the problem is downstream.
Check your email provider dashboard
Most transactional email providers (SendGrid, Mailgun, AWS SES, Postmark, Resend) have dashboards that show:
- Delivery attempts and status codes
- Bounces and bounce reasons
- Spam complaints
- Throttling or rate limiting events
- Authentication failures (SPF/DKIM/DMARC)
If the provider shows a bounce, the bounce reason is your best diagnostic clue. Common bounce reasons include:
550 5.7.1— Recipient address rejected (disposable domain, invalid address)550 5.1.1— Recipient address not found550 4.2.1— Inbox full550 5.7.1— IP address blocked
Verify DNS and authentication records
Use tools like MXToolbox or dig to verify your SPF, DKIM, and DMARC records are correctly published and pass validation. Authentication failures are a common cause of spam filtering.
Test with different recipients
If email to one recipient fails but others succeed, the issue may be specific to that recipient's mail server or policy. Test with multiple recipient types (Gmail, Outlook, corporate mail) to isolate the problem.
Test spam filtering
Use tools like GlockApps, Mail-Tester, or your provider's spam checker to test how your email is scored. These tools analyze your content, authentication, and reputation to predict whether messages will be marked as spam.
Recipient inbox checks
When debugging with a real recipient, ask them to check:
- Is the message in spam or junk folders?
- Are any email filters or rules diverting the message? li>Is the inbox full or quota-exceeded?
- Are there auto-responder rules that triggered?
Using temporary email for debugging
Temporary email is excellent for manual debugging because:
- It provides a clean external recipient with no existing filters or rules
- You can generate a fresh address for each test without polluting your real inbox
- It blocks remote images, so you can see how email renders without tracking distractions
- It shows whether your email is being delivered at all, independent of recipient-side filtering