Bounce codes
Hard bounces (permanent)
| Code | Meaning | Fix |
|---|---|---|
| 550 5.1.1 | Recipient address does not exist | Remove the address from your list. Verify spelling |
| 550 5.1.2 | Domain does not exist | The domain is invalid or has been deactivated. Remove the contact |
| 550 5.1.3 | Invalid mailbox syntax | The email address format is wrong (missing @, etc.) |
| 550 5.2.1 | Mailbox disabled or no longer active | Remove the address. The account has been closed |
| 550 5.7.1 | Message rejected by policy | The receiving server permanently rejected your message. Check content and authentication |
| 550 5.4.1 | Relay not allowed | The receiving server does not accept mail for this address |
Soft bounces (temporary)
| Code | Meaning | Fix |
|---|---|---|
| 421 4.7.0 | Connection rate limited | Too many connection attempts. Reduce send frequency |
| 450 4.2.1 | Mailbox full | Recipient’s inbox is full. Will be retried automatically |
| 451 4.3.0 | Server temporarily unavailable | Mail server is down. Retried automatically |
| 452 4.5.3 | Too many recipients | Reduce the number of recipients per email batch |
| 451 4.7.1 | Greylisting | The server is temporarily delaying delivery. Automatic retry will succeed |
Authentication failures
| Error | Meaning | Fix |
|---|---|---|
| SPF fail (550 5.7.23) | Your sending IP is not in the SPF record | Add the HoopAI SPF include to your DNS. See Email deliverability guide |
| DKIM fail (550 5.7.20) | DKIM signature verification failed | Verify DKIM CNAME records in DNS. Ensure they match the records in Settings → Email Services |
| DMARC fail (550 5.7.26) | Email failed DMARC alignment | Ensure both SPF and DKIM are properly configured. Check that your From address matches the authenticated domain |
| No authentication | Missing SPF/DKIM/DMARC records | Set up a dedicated sending domain in Settings → Email Services |
Blacklist and reputation errors
| Error | Meaning | Fix |
|---|---|---|
| 550 5.7.1 Blacklisted | Your sending IP or domain is on a blocklist | Check MXToolbox for blacklist status. Request delisting. Review sending practices |
| 550 Sender reputation too low | ISP has flagged your domain due to poor reputation | Reduce volume, improve engagement, warm up the domain. See Email deliverability guide |
| 550 JunkMail rejected | Content was flagged as junk by the receiving server | Review subject line and body for spam triggers. Remove excessive links and images |
Rate limiting errors
| Error | Meaning | Fix |
|---|---|---|
| 421 Too many connections | Sending too fast to this server | HoopAI automatically throttles. No action needed |
| 452 Too many messages | Exceeded the receiving server’s hourly/daily limit | Spread sends over a longer period. Avoid blasting all at once |
| 550 Daily sending limit exceeded | Your account hit the daily sending cap | Check your plan limits in Settings → Billing. Upgrade or spread sends across days |
Gmail-specific errors
| Error | Meaning | Fix |
|---|---|---|
| 550 5.7.26 Unauthenticated email | Gmail requires SPF and DKIM for bulk senders | Configure dedicated domain with SPF + DKIM |
| 550 5.7.1 Spam detected | Gmail classified your email as spam | Review content, reduce link density, improve subject line. Check Google Postmaster for domain reputation |
| 421 4.7.28 Rate limit exceeded | Gmail is throttling your sends | Reduce sending speed. Warm up domain gradually |
| 550 5.7.27 SPF/DMARC required | Gmail enforcing 2024+ sender requirements | Set up SPF, DKIM, and DMARC. See Sender requirements |
Outlook/Microsoft errors
| Error | Meaning | Fix |
|---|---|---|
| 550 SC-001 | IP blocked due to spam reports | Apply for delisting at sender.office.com. Reduce complaint rate |
| 550 DY-001 | Content flagged as spam | Review content. Remove suspicious URLs and attachments |
| 421 RP-001 | Rate limited by Microsoft servers | Reduce send rate. Spread sends over time |
Troubleshooting steps
Check the email log
Go to Conversations or Reporting → Email Reporting and find the failed email. Click to view the error code.
Verify DNS authentication
In Settings → Email Services → Dedicated Domain, confirm that SPF, DKIM, and DMARC show a green checkmark.
Check domain reputation
Use Google Postmaster Tools and MXToolbox to check your domain and IP reputation.
Review content
Remove spam triggers from your email subject and body. Test with mail-tester.com before sending campaigns.