Email Bounce Codes Reference
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 |
Authentication-Related
| 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 |
Reputation-Related
| 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.