How do I debug email delivery problems?

A good way to debug email delivery problems is to trace the message end-to-end and identify where it stops: app sending, SMTP handoff, receiving server acceptance, or inbox placement.

1) Confirm the message was actually sent

Check your app logs for:

• SMTP response codes
• Message ID / queue ID
• Any exceptions or retries
• Timestamp of send attempt

If you use a provider (SendGrid, SES, Mailgun, Postmark, etc.), check their logs/events first.

2) Verify basic configuration

Common issues:

• Wrong SMTP host/port
• Authentication failure
• TLS/SSL mismatch
• Wrong “From” address or domain
• Missing DNS records: SPF, DKIM, DMARC
• Reverse DNS / PTR missing for self-hosted mail servers

3) Check the receiving side

Find out whether the receiving server:

• Accepted the message but filtered it to spam/quarantine
• Rejected it during SMTP
• Deferred it temporarily (greylisting, rate limiting, reputation issues)

Look for SMTP status codes:

• 2xx = accepted
• 4xx = temporary failure, usually retry
• 5xx = permanent failure

4) Inspect the full email headers

If the email arrived somewhere, view the headers and look for:

• Received: chain
• Authentication results: SPF=pass/fail, DKIM=pass/fail, DMARC=pass/fail
• Spam filter clues
• Original sender IP and domain

5) Test deliverability with a known-good mailbox

Send to:

• Gmail
• Outlook/Hotmail
• Yahoo
• A corporate mailbox if available

Compare results across providers. A message landing in one inbox but not another often points to reputation or authentication issues.

6) Check reputation and blocklists

If mail is accepted but not delivered:

• Sender IP/domain reputation may be poor
• You may be on a blocklist
• Content may look spammy
• You may be sending too fast or to bad addresses

Use provider dashboards and blocklist lookup tools.

7) Validate content

Email content can trigger filtering:

• Too many links
• URL shorteners
• Spammy wording
• Missing plain-text part
• Broken HTML
• Attachment issues

8) Test with a minimal message

Send a plain-text email with:

• Short subject
• No links
• No attachments
• Simple body

If that works, the issue is likely content or rendering-related.

9) Use mail testing tools

Helpful tools:

• swaks for SMTP testing
• openssl s_client for TLS checks
• Mail tester services
• Provider-specific logs and event webhooks

10) If you run your own mail server

Check:

• SMTP daemon logs
• Queue status
• Deferred mail
• DNS resolution
• PTR/rDNS
• Port 25 connectivity
• SpamAssassin/Rspamd logs
• Postfix/Exim/Exchange logs

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Quick troubleshooting checklist

• Can you authenticate to SMTP?
• Is the message accepted by the SMTP server?
• Do SPF/DKIM/DMARC pass?
• Is the sender IP/domain reputable?
• Is the message landing in spam/quarantine?
• Are there provider or server-side logs showing rejection?

If you want, I can help you debug a specific setup—like Gmail not receiving from Postfix, SendGrid/SES issues, or SMTP error codes.