Email Bounce Codes Reference
A bounce code is what the receiving server said when it would not take your email. This page explains what each one means, whether it is temporary or permanent, how the message reads through each of the main send integrations, and what to do about it.
Quick Diagnostic Flowchart
- All sends failing? Check Send Integration status and API key: Send Integrations > click integration > Run Test
- Specific domains bouncing? Check DNS authentication: Settings > Sender Domains > verify SPF, DKIM, DMARC
- Random contacts bouncing? Check bounce codes in the tables below
- Landing in spam? Check sender reputation via ESP dashboard and review email content
Soft Bounces (Temporary - 4xx)
| Code |
Meaning |
Action |
| 421 |
Service not available |
Retry later, server is temporarily overloaded |
| 450 |
Mailbox unavailable |
Recipient's mailbox is temporarily locked or full |
| 451 |
Local error in processing |
Server-side issue, retry later |
| 452 |
Insufficient storage |
Recipient's mailbox is full |
| 421 4.7.0 |
Too many connections from IP |
Reduce sending rate; check Volume Caps on integration |
Hard Bounces (Permanent - 5xx)
| Code |
Meaning |
Action |
| 550 |
Mailbox not found |
Remove this address from your list — it does not exist |
| 551 |
User not local |
Address is not on this server, may need forwarding |
| 552 |
Message too large |
Reduce email size (images, attachments) |
| 553 |
Invalid address format |
Check the email address for typos |
| 554 |
Transaction failed |
Generic rejection — check sender reputation and content |
| 550 5.1.1 |
Recipient address rejected |
Address does not exist; remove from list |
| 550 5.1.2 |
Host not found |
Domain does not exist; remove from list |
| Code |
Meaning |
Action |
| 550 5.7.1 |
SPF check failed |
Fix SPF DNS record: Sender Domains > check SPF status |
| 550 5.7.1 |
DKIM signature invalid |
Verify DKIM DNS record: Sender Domains > check DKIM status |
| 550 5.7.1 |
DMARC policy reject |
Align SPF and DKIM with DMARC policy: Sender Domains > check DMARC status |
| Code |
Meaning |
Action |
| 550 5.7.1 |
Blocked by policy |
IP or domain is blocklisted — check blocklist status via ESP dashboard |
| 550 |
Spam content detected |
Review email content for spam triggers |
| 421 |
Too many connections |
Reduce sending rate or configure Volume Caps on the Send Integration |
Common ESP Error Messages
SendGrid
| Message Pattern |
Meaning |
Action |
| "The from address does not match a verified Sender Identity" |
Sender not verified in SendGrid |
Verify domain or sender in SendGrid dashboard |
| "You have reached your daily sending limit" |
Plan limit hit |
Upgrade SendGrid plan or wait for reset |
| "sandboxmode is only allowed over https" |
API misconfiguration |
Check integration API settings |
Postmark
| Message Pattern |
Meaning |
Action |
| "Inactive recipient" |
Address previously hard bounced in Postmark |
Reactivate in Postmark or remove from list |
| "Not allowed to send" |
Server not approved for sending |
Check Postmark server approval status |
| "Invalid 'From' address" |
From address not configured |
Add sender signature in Postmark |
Brevo (Sendinblue)
| Message Pattern |
Meaning |
Action |
| "unauthorized: Key not found" |
Invalid API key |
Regenerate API key in Brevo dashboard |
| "blocked: Contact is blocked" |
Contact on suppression list |
Check Brevo suppression list or remove block |
| "hard_bounce" |
Permanent delivery failure |
Remove address; check Delivery Events for details |
AWS SES
| Message Pattern |
Meaning |
Action |
| "Email address is not verified" |
Sender not verified in SES |
Verify domain or address in AWS SES console |
| "Daily sending quota exceeded" |
Account limit reached |
Request limit increase in AWS SES |
| "MessageRejected: Account is in sandbox" |
SES sandbox mode |
Request production access from AWS |
What To Do
- Soft bounces: Usually resolve automatically. Monitor via Delivery Events and retry. If a contact soft-bounces repeatedly (3+ times), consider removing them.
- Hard bounces: Remove the address immediately to protect sender reputation. Navigate to Delivery Events > filter by "HardBounced" > export and suppress those contacts.
- Authentication failures: Fix DNS records. Go to Sender Domains > click the affected domain > follow the DNS instructions for SPF, DKIM, and DMARC.
- Reputation issues: Review sending practices, clean your list regularly, and check blocklist status. Consider setting up a Bounce Rule at Delivery Events > Bounce Rules to auto-suppress addresses matching specific bounce patterns.
- ESP outages: Check your ESP's status page. If there is an active incident, wait for resolution and monitor Delivery Events for recovery.
