Email error codes

When an email fails to deliver, HoopAI logs the error from the receiving mail server. Use this reference to diagnose and resolve issues.

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550 5.2.1	Mailbox disabled or no longer active	Remove the address.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.

Contact support

If errors persist after trying the above, contact HoopAI support with the full error code and email details.

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Bounce codes

Hard bounces (permanent)

Soft bounces (temporary)

Authentication failures

Blacklist and reputation errors

Rate limiting errors

Gmail-specific errors

Outlook/Microsoft errors

Troubleshooting steps

​Bounce codes

​Hard bounces (permanent)

​Soft bounces (temporary)

​Authentication failures

​Blacklist and reputation errors

​Rate limiting errors

​Gmail-specific errors

​Outlook/Microsoft errors

​Troubleshooting steps

Bounce codes

Hard bounces (permanent)

Soft bounces (temporary)

Authentication failures

Blacklist and reputation errors

Rate limiting errors

Gmail-specific errors

Outlook/Microsoft errors

Troubleshooting steps