Email Bounce Code Reference Guide
Understanding Email Bounces
When sending emails from NationBuilder, receiving servers may return bounce codes if delivery fails. Understanding these codes helps you keep your email list healthy and improve deliverability.
Quick Reference:
🔴 Hard Bounce = Permanent failure → Remove supporter from your lists
🟠 Soft Bounce = Temporary issue → Retry, then remove if persistent
Some Common Bounce Codes
Bounced Address
This is a "generic" bounce error that needs to be reviewed and cleared by our support team. Please reach out to [email protected] for next steps.
If you have multiple recipients showing this error, use the drop-down menu in the top right corner to “Add to a list.” Once the list is created, send our support team the link to the list so we can review all affected emails and help clear the generic bounce errors more efficiently.
550 - Mailbox Not Found / Unavailable
Type: Hard Bounce
What it means: The recipient's email address doesn't exist or has been deleted. This is one of the most common hard bounces.
What to do:
Check for typos in the email address
Verify the person hasn't changed organizations or email providers
Remove this email address from your list if you aren't able to verify it
Keeping invalid addresses hurts your sender reputation
5.1.1 - Bad Destination Mailbox Address
Type: Hard Bounce
What it means: The email address does not exist. This is the enhanced SMTP version of a 550 error with more specificity.
What to do:
Double-check the email address for typos
Update your database if you have an alternate contact method
Remove this contact from your mailing list
554 - Message Rejected / Suspected Spam
Type: Hard Bounce
What it means: The recipient's server rejected the message, often due to spam suspicion or your IP being blacklisted.
What to do:
Review your email content for spam triggers
Ensure proper SPF, DKIM, and DMARC authentication
Consider warming up your sending domain if it's new
Reach out to support, [email protected], to review the listed IP address and email content
452 - Mailbox Full (Temporary)
Type: Soft Bounce
What it means: The recipient's mailbox has exceeded its storage quota. This is a temporary issue that may be resolved.
What to do:
Retry sending after 24-48 hours
If it persists after multiple attempts, consider removing the email address
Some users never clear their mailboxes, so monitor over time
After 5 - 7 attempts, consider verifying the address or replacing it with an alternative
421 - Service Temporarily Unavailable
Type: Soft Bounce
What it means: The recipient's mail server is temporarily down or experiencing issues.
What to do:
NationBuilder will automatically retry delivery
No action needed unless it persists beyond 72 hours
If multiple recipients show this, it is likely a server-wide outage
450 - Mailbox Temporarily Unavailable
Type: Soft Bounce
What it means: The mailbox is temporarily unavailable, possibly due to maintenance or being accessed.
What to do:
Allow automatic retry - usually resolves within hours
If persistent beyond 48 hours, investigate further
May indicate the user is performing account maintenance
Complete Hard Bounce Code List
553 - Invalid Mailbox Name
What it means: The email address format is incorrect, or the mailbox name doesn't follow proper syntax rules.
What to do:
Remove or correct the email address
Check for special characters or formatting issues
Validate email addresses at collection time to prevent this
5.7.1 - Delivery Not Authorized / Message Refused
What it means: The recipient's server refused to accept mail from your sending address due to security policies.
What to do:
Verify your domain authentication (SPF, DKIM, DMARC).
Check whether your domain is on the recipient’s internal blocklist.
Ensure your sending practices comply with standard email guidelines.
Contact the recipient’s IT department if this is legitimate business correspondence.
521 - Domain Not Accepting Email
What it means: The recipient’s domain is not accepting email.
What to do:
Verify the domain is still active
Remove all contacts from this domain
Check if the organization has migrated to a new domain
551 - User Not Local / Try Forwarding
What it means: The user doesn't exist on this server. The email may have been forwarded elsewhere.
What to do:
Remove this address from your list
Try to find an updated email address for the contact
The forwarding address (if any) is not provided for privacy
5.1.2 - Bad Destination System Address
What it means: The domain name in the email address is invalid or doesn't exist.
What to do:
Check for typos in the domain name (e.g., 'gmial.com' instead of 'gmail.com')
Verify the domain exists using a DNS lookup
Remove this email address
5.1.3 - Bad Destination Mailbox Address Syntax
What it means: The email address format is structurally incorrect (missing '@', invalid characters, etc.).
What to do:
Review and correct the email address format
Implement email validation at signup to prevent this
Remove if the correct format cannot be determined
5.2.1 - Mailbox Disabled or Not Accepting Mail
What it means: The recipient's mailbox exists but has been disabled or set to not accept incoming messages.
What to do:
Remove from active lists
The account may have been deactivated due to inactivity
Try alternate contact methods if available
5.4.4 - Unable to Route
What it means: The mail server cannot determine how to route the message to the destination.
What to do:
Check the email address and domain for errors
Verify DNS records for the domain are configured properly
This may indicate a misconfigured recipient server
5.7.1 - Relay Access Denied
What it means: The recipient server won't relay messages from your server, often a security configuration.
What to do:
Ensure your sending domain is properly authenticated
Check with NationBuilder support about your sending configuration
Verify you're not accidentally sending through an unauthorized relay
Complete Soft Bounce Code List
4.2.2 - Mailbox Full
What it means: Enhanced SMTP code indicating the recipient's inbox has reached storage limit.
What to do:
Retry in 24-48 hours
Monitor this contact - chronic full mailboxes may indicate inactive users
Consider moving to a re-engagement campaign
Treat as hard bounce if persists beyond 7 days
422 - Recipient's Mailbox Full
What it means: Traditional SMTP code for a full mailbox. Similar to 452 but less commonly used.
What to do:
Wait 24-48 hours before retry
Track frequency - repeatedly full mailboxes suggest disengagement
431 - Server Error
What it means: The receiving server encountered an internal error while processing the message.
What to do:
Allow automatic retry
Usually resolves quickly without intervention
If widespread across a domain, the issue is on their end
441 - Recipient Server Not Responding
What it means: The destination server is not responding to connection attempts.
What to do:
Automatic retry will handle this
Typically resolves within a few hours
May indicate network issues on the recipient's end
442 - Connection Dropped
What it means: The connection to the recipient's server was dropped during transmission.
What to do:
NationBuilder will automatically retry
Usually, a transient network issue
No action needed from you
447 - Message Timed Out
What it means: The outgoing message spent too long in the incoming server's queue.
What to do:
Allow retry - often resolves automatically
May indicate high volume at the recipient's server
Consider sending at off-peak times if persistent
451 - Local Processing Error
What it means: The recipient's server had a temporary processing error.
What to do:
Automatic retry will handle this
No action needed unless it persists
Issue is on the recipient's server side
4.3.2 - Mail System Too Busy
What it means: The recipient's mail server is overloaded and temporarily cannot process messages.
What to do:
Allow automatic retry during off-peak hours
The recipient's organization may be experiencing high email volume
Usually resolves within hours
4.4.1 - Network Connection Issue
What it means: There's a temporary network problem between mail servers.
What to do:
NationBuilder will retry automatically
Typically resolves quickly
No action needed from you
4.7.1 - Message Temporarily Delayed
What it means: The message is delayed due to recipient server policy or security scanning.
What to do:
Allow automatic retry
Message may be in security quarantine
Usually delivers within 24 hours
If persistent, may need to verify your sending reputation
Best Practices for Managing Bounces
For Hard Bounces:
Remove email address - Avoid sending to invalid addresses; it hurts your sender reputation.
Clean regularly - Set up a process/workflow to review and remove hard-bounced email addresses
Validate at signup - Use email validation tools to catch bad addresses early
Monitor bounce rates - High rates (>3%) can hurt your sender reputation
For Soft Bounces:
Give it time - Allow 2-3 retry attempts over 48-72 hours
Convert to hard - If a soft bounce persists, treat it as hard
Watch patterns - Repeatedly full mailboxes often indicate disengaged users and/or an outdated mailing list
Re-engagement - Consider a win-back campaign before removing
Maintaining Sender Reputation:
Keep bounce rate below 3%
Review hard bounces before next send
Ensure SPF, DKIM, and DMARC are properly configured
Do not purchase, rent, or scrape email lists
Use double opt-in for new subscribers, when possible
Monitor blacklist status regularly
When to Contact Support
Contact NationBuilder support if you experience:
Sudden spike in bounce rates across multiple campaigns
Widespread 554 errors (may indicate a deliverability issue)
Bounces affecting entire domains you're sending to
Authentication-related bounces (5.7.1) for verified domains
Questions about your sending reputation or IP status
Additional Resources
Last Updated: [Current Date]
Questions? Contact NationBuilder Support