Persistent bouncing is a signal that something needs attention. The pattern of bounces helps identify the cause. Here is what to look for and how to fix common problems.
Cause 1: Invalid Email Addresses
The most common bounce cause is simply that the email address does not exist.
How This Happens
- Typos at signup (gmial.com instead of gmail.com)
- People entering fake addresses
- Employees leaving companies (address deleted)
- Using old lists with abandoned addresses
How to Fix
- Remove hard bounced addresses immediately
- Use double opt-in to verify addresses at signup
- Validate email syntax on your signup forms
- Run existing lists through verification services
Also in this series
- For what a bounce back actually is, hard versus soft, and how to read the SMTP code, see What Causes Email Bounce Backs?.
- For the step-by-step fix once you know the cause, see How Do I Fix Email Delivery Failures?.
Cause 2: Full Mailboxes
Some email accounts reach storage limits and cannot accept new messages.
Signs of This Problem
- Soft bounce codes (452) mentioning storage
- Bounces for addresses that previously worked
- Temporary nature of the failure
How to Handle
- Allow automatic retries for soft bounces
- Track addresses with repeated full mailbox errors
- Remove after 3-5 consecutive failures
Cause 3: Sender Reputation Problems
When your reputation is damaged, receiving servers reject your mail even when the addresses are valid.
Signs of Reputation Issues
- Bounces with "blocked," "denied," or "policy" in the message
- Bounces affecting valid, engaged recipients
- Problems concentrated at specific providers
How to Fix
- Check Google Postmaster Tools for domain reputation
- Review Sender Score for IP reputation
- Check for blocklist presence
- Reduce sending volume and focus on engaged subscribers
- Rebuild reputation over 2-4 weeks
Cause 4: Authentication Failures
Missing or misconfigured authentication causes rejections at strict providers.
Signs of Authentication Issues
- Bounce messages mentioning SPF, DKIM, or DMARC
- Bounces that started after DNS changes
- Higher bounce rates at Gmail or Yahoo specifically
Check Your Authentication
Send a test email to yourself at Gmail. Open the email, click the three dots, select "Show original," and search for "Authentication-Results." All three protocols (SPF, DKIM, DMARC) should show "pass."
How to Fix
- Verify SPF record includes all sending services
- Confirm DKIM is properly configured and signing
- Ensure DMARC is published and aligned
Cause 5: Server or Network Issues
Sometimes the problem is temporary infrastructure failure.
Signs of Server Issues
- Soft bounce codes (4xx) mentioning "temporary" or "try again"
- Bounces that resolve when retried
- Affecting random recipients, not patterns
How to Handle
- Allow your system to automatically retry
- Most server issues resolve within 24-72 hours
- If persistent, check recipient domain status
Diagnosing Your Specific Problem
Look for Patterns
- All bounces from one domain? That domain may be blocking you
- All bounces have the same error? Systematic issue (authentication, blocklist)
- Bounces are random addresses? Likely invalid addresses
- Bounces started suddenly? Recent change triggered it (DNS, reputation event)
Check the Bounce Codes
- 5xx codes: Permanent failures requiring removal
- 4xx codes: Temporary failures that may resolve
High Bounce Rates Are Dangerous
Bounce rates above 2-3% damage your sender reputation. If you are seeing high bounces, pause sending until you diagnose and fix the issue. Continuing to send with high bounces makes the problem worse.