Email bounces occur when messages can't be delivered.
Soft bounces (temporary failures like full inbox, server issues, content blocks) may resolve themselves.
Hard bounces (permanent failures like invalid addresses, typos, closed accounts) require immediate action.
Fix by: Verify email addresses before sending, clean your list regularly, monitor blacklists, check DNS settings (SPF, DKIM, DMARC), avoid spam-like content, and limit daily volume to 200 emails. Use email verification tools and pause campaigns if bounce rate spikes.
Symptoms
Campaign reports show high bounce rate (over 5%)
Emails marked as Bounced in campaign activity
Error codes in bounce messages (550, 554, 421, etc.)
Soft bounce warnings for temporary delivery failures
Hard bounce errors for invalid or non-existent addresses
Recipients report not receiving emails
Environment
Applies to all email campaigns in lemlist. High bounce rates damage the sender's reputation and affect deliverability for future campaigns. Lemlist automatically pauses campaigns with a bounce rate exceeding 50% if a minimum of 100 leads have been contacted, helping to protect sender reputation.
Understanding Bounce Types
Soft Bounce (Temporary)
Causes:
Recipient inbox full
Temporary server problems
Email size limit exceeded
Content blocked by filters
DMARC or antivirus requirements not met
Greylisting or throttling by the recipient server
DNS misconfiguration
Antispam filters are blocking unverified senders
Provider-specific limits and reputation issues
High email volume or sudden spikes
Recipient servers blocking IP addresses
Filtering by anti-spam tools such as Cloudmark
Action: Monitor soft bounces. May resolve automatically, but recurring soft bounces become hard bounces.
Hard Bounce (Permanent)
Causes:
Invalid or non-existent email address
Typos in email address
Outdated addresses or closed accounts
Catch-all domains are rejected after verification
Mailbox deactivated or deleted after verification
Blocks due to provider-specific reputation-based evaluations
Action: Remove immediately to protect the sender's reputation.
Step-by-Step Fix
1. Pause campaigns if the bounce rate is high
If bounce rate exceeds 5%:
Pause active campaigns
Assess damage before sending more emails
Prevents further reputation damage
✅ Verify: Campaigns paused, no more emails being sent
Keep in mind that in lemlist bounce rate alerts are triggered at 10%, and campaigns are automatically paused at 50% if at least 100 leads have been contacted.
2. Verify DNS authentication settings
Go to your domain DNS provider
Confirm these records are properly configured:
SPF record - Authorizes sending servers
DKIM record - Email signature verification
DMARC record - Policy for failed authentication
Use tools to verify:
MXToolbox DNS lookup
Google Admin Toolbox Check MX
✅ Verify: All three records present and valid
3. Clean email list
Remove or fix:
Hard bounced addresses (invalid/non-existent)
Repeated soft bounce addresses
Addresses with typos (common: gmial.com, yahooo.com)
Inactive addresses (no engagement in 6+ months)
Use the email verification tool before sending:
Verify addresses are deliverable
Remove catch-all and risky addresses
Clean the list before importing to the campaign
Use lemlist enrichment for additional email address verification.
✅ Verify: List cleaned, verification tool shows addresses are valid
4. Check blacklist status
Use these tools to check the domain/IP:
MXToolbox Blacklist Check
Spamhaus
Microsoft SNDS (for Outlook deliverability)
Google Postmaster Tools
If blacklisted:
Follow the delisting process for the specific blacklist
Fix the underlying issue (spam complaints, authentication)
Request removal
✅ Verify: Domain and IP not on major blacklists
5. Optimize email content
Avoid spam triggers:
Excessive caps or exclamation marks
Spam phrases ("FREE", "Act now", "Limited time")
Too many links or images
Misleading subject lines
Large attachments
Low personalization in content
Overly repetitive content
Use spam checker tools before sending
✅ Verify: Spam score low (under 5)
6. Monitor sender reputation
Check reputation using:
Microsoft SNDS (Outlook reputation)
Google Postmaster Tools (Gmail reputation)
Sender Score (overall reputation)
Improve reputation by:
Sending to engaged recipients only
Maintaining a low bounce rate (under 3%)
Avoiding spam complaints
Consistent sending patterns
✅ Verify: Reputation score good (green/high rating)
7. Control email volume
Best practices:
Send a maximum of 200 emails per day per account
Gradually increase volume (warm up)
Spread sends throughout day
Use inbox rotation for larger volumes
✅ Verify: Daily volume within safe limits
Confirm It's Fixed
✓ Bounce rate drops below 5% (ideally under 3%)
✓ Hard bounces removed from list
✓ DNS records (SPF, DKIM, DMARC) verified and valid
✓ Domain not on blacklists
✓ Sender reputation score improving
✓ Test emails deliver successfully
Why It Happens
Bounces occur from invalid addresses, poor list hygiene, DNS authentication issues, spam-like content, or damaged sender reputation. Soft bounces may be temporary (full inbox, server issues) while hard bounces are permanent (invalid addresses). High bounce rates signal to providers that you're sending to bad lists, damaging your reputation. Even after verification, addresses may become invalid or encounter deliverability issues such as temporary server rejections or recipient-side filtering. Occasionally, unseen bounces can result from lead updates or removals in the system after initial sending metrics were logged.
Prevent this: Verify emails before sending, clean lists regularly, maintain proper DNS authentication, avoid spam content, control sending volume, and monitor reputation.
Prevention Checklist
✓ Set up custom tracking domain - Improves deliverability. This helps enhance the sender's reputation and establishes trust with recipient servers.
✓ Verify email addresses - Use the verification tool before importing
✓ Configure DNS properly - SPF, DKIM, DMARC records
✓ Monitor domain health - Regular MXToolbox checks
✓ Use lemwarm - Warm up new domains
✓ Limit daily volume - Stay under 200 emails per day
✓ Avoid spam triggers - Clean, professional content
✓ Clean lists regularly - Remove bounces and inactive addresses
✓ Monitor blacklists - Check weekly