Why aren't my transactional emails being delivered?

Transactional emails can fail to deliver for a bunch of reasons, and the fastest way to narrow it down is to check where they’re getting stuck: your app, your email provider, or the recipient mailbox.

Common causes

1. Bad sender authentication

   • Missing or incorrect SPF, DKIM, or DMARC
   • Sending from a domain that isn’t properly verified

2. IP/domain reputation issues

   • New sending domain/IP with no reputation
   • Previous spam complaints or bounces
   • Shared IP reputation problems

3. Suppression or blocklists

   • The recipient address is on your provider’s suppression list
   • Your sending domain/IP is on a blocklist

4. Invalid recipient or mailbox issues

   • Typos in email addresses
   • Recipient mailbox full
   • Recipient server rejecting the message

5. Content or formatting problems

   • Spammy subject lines or wording
   • Broken headers, malformed MIME, bad links
   • Missing plain-text part

6. Rate limiting or throttling

   • Sending too many emails too quickly
   • API/webhook retries causing duplicate bursts

7. Wrong environment/config

   • Emails going to a test inbox, sandbox, or disabled in production
   • Incorrect SMTP/API credentials
   • Using the wrong from-address or region/provider endpoint

8. Recipient-side filtering

   • Email delivered to spam/junk/promotions
   • Corporate filters silently quarantining mail

What to check first

• Look at the email provider logs for a bounce, deferral, or rejection reason
• Confirm SPF/DKIM/DMARC are passing
• Send a test email to:
  • Gmail
  • Outlook/Hotmail
  • A corporate address
• Check whether mail is landing in spam/junk
• Verify the provider’s suppression/bounce list
• Review the exact SMTP response or API error code

If you want to diagnose it quickly

Reply with:

• Which provider you use (SES, SendGrid, Mailgun, Postmark, SMTP server, etc.)
• The exact error or bounce message
• Whether messages are not sent, sent but bounced, or sent but not seen
• Your domain setup status for SPF/DKIM/DMARC

If you paste a bounce log or SMTP response, I can help interpret it.