Why Bounce Rate Increased: Causes and Solutions

Diagnosis

Check 1: Did You Import New Contacts?

The most common cause of sudden bounce rate increases. If you recently imported contacts:

1. Check the bounce rate specifically for the imported segment
2. Remove all hard bounces
3. Validate remaining imported addresses through ZeroBounce/NeverBounce
4. Implement validation before import as a permanent process

Check 2: Has Your List Been Cleaned Recently?

Email lists decay at 2-3% per month (people change jobs, abandon addresses, domains expire). If your list hasn't been cleaned in 6+ months:

1. Run the full list through an email validation service
2. Remove all invalid, catch-all, and role-based addresses
3. Suppress contacts with no engagement in 180+ days
4. Implement quarterly cleaning

Check 3: Did DNS Records Change?

A DNS change can break authentication, causing some servers to bounce instead of delivering:

1. Check SPF record is intact and includes all senders
2. Verify DKIM records are published and correct
3. Confirm DMARC is published
4. Use MXToolbox to verify all records

Check 4: Did You Change ESPs?

ESP migration can cause bounces if:

• Old suppression lists weren't migrated (previously bounced addresses get retried)
• New ESP uses different authentication (DKIM selector changed)
• New ESP's IPs have different reputation

Check 5: Are You Sending to a Purchased List?

Purchased lists have the highest bounce rates (often 10-30% invalid). If someone on your team purchased a list:

1. Stop sending to it immediately
2. Never use purchased lists again
3. Clean and validate if you must use contacts from it
4. Monitor for blacklisting (purchased lists contain spam traps)

Recovery

Immediate Actions

1. Remove all hard bounces from your list/ESP immediately
2. Pause campaigns to the problematic segment
3. Check blacklists (high bounces can trigger listings)
4. Validate remaining addresses through email validation service

Short-Term (1-2 Weeks)

1. Send to engaged-only segment (opened in last 30 days)
2. Monitor bounce rate on every send — must stay under 2%
3. Check reputation in Google Postmaster Tools and SNDS
4. Review signup sources — where are invalid addresses coming from?

Long-Term Prevention

1. Real-time validation on all signup forms (API integration)
2. Quarterly list cleaning through validation service
3. Never import without validating — make this a process rule
4. Sunset policy: Remove contacts with no engagement in 180 days
5. Double opt-in for high-risk sources (contests, giveaways)

ESP Bounce Rate Thresholds

ESP	Warning Threshold	Suspension Threshold
Mailgun	5%	10%
SendGrid	5%	10%
AWS SES	5%	10% (auto-suspension)
Klaviyo	Internal monitoring	Account review
Mailchimp	Warning at 3%+	Account suspension possible

AWS SES is the strictest — automatic account suspension at 10% bounce rate with no warning. Monitor CloudWatch alarms.

Practitioner note: The pattern I see most often: company imports a "leads list" from a webinar partner or event. 20% of the list bounces. The ESP flags the account. Reputation drops. All sending (including to clean contacts) is now degraded. Validate EVERY list before importing — even if it comes from a "trusted" source.

Practitioner note: If your bounce rate is above 5% and you can't figure out why, check for one specific thing: are you sending to suppressed/bounced contacts that your previous ESP had flagged? When migrating ESPs, export and migrate your suppression list alongside your active list. Otherwise, the new ESP retries addresses the old one already knew were dead.

If your bounce rate is high and you need a recovery plan, schedule a consultation — I'll diagnose the cause and implement list cleaning automation.

Sources

• Google: Bounce Rate Guidelines
• AWS: SES Bounce Handling

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v1.0 · March 2026