Even with properly configured domains and mailboxes, cold emails can still land in spam. Modern ESPs like Gmail, Outlook, and Yahoo evaluate many signals before deciding whether an email belongs in inbox or spam.
Spam placement is usually caused by a combination of:
Mailbox reputation
Domain reputation
Sending behavior
Email content
Recipient engagement
Technical setup
In most cases, it is not caused by a single issue.
New mailboxes have no reputation. If you start campaigns too aggressively before warming up, ESPs may flag your emails as suspicious.
Fix
Warm up new mailboxes for 2 weeks with warm-up emails only and no cold emails. Warm-up starts at around 3-4 emails per day and reaches 20 per day by the end of week 2
Keep warm-up enabled continuously, also after you start sending cold emails
Increase cold sending volume gradually, up to a maximum of 30 cold emails per day per mailbox
Pre-warmed mailboxes still require ongoing warm-up activity. For the full timeline, see Cold Email Warm-Up Best Practices.
This is one of the most common causes. Even with healthy infrastructure, repetitive or overly salesy copy can trigger spam filters.
Common causes
Repetitive messaging
Too many links
Aggressive sales language
Overused signatures
Lack of personalization
Heavy formatting or images
Fix
Use personalization and spintax
Keep emails short and natural
Reduce links and tracking
Rotate signatures occasionally
Write conversational emails
If your email contains blacklisted links, ESPs may instantly send the message to spam.
Fix
Remove links and test again
Avoid URL shorteners
Use custom tracking domains
Check domains using blacklist tools
Useful tools
MXToolbox Blacklist Check
Mailmeteor Spam Checker
Mail Tester
Sudden spikes in sending volume are a major red flag.
Fix
Scale slowly over time, and stay within 30 cold emails per day per mailbox. Example for a new mailbox:
Weeks 1-2: warm-up only, ramping from 3-4 up to 20 warm-up emails per day, no cold emails
Week 3: start with 15-20 cold emails per day, with warm-up still running
Week 4: build up to 30 cold emails per day, plus 20 warm-up emails (50 in total)
Avoid aggressive scaling on new domains. To send more, add more mailboxes and domains instead of increasing the volume per mailbox.
If recipients ignore, delete, or mark your emails as spam, your reputation declines.
Fix
Improve targeting
Personalize emails deeply
Focus on reply rates instead of volume
Missing or incorrect DNS records can hurt deliverability. The important records are SPF, DKIM and DMARC.
Fix
Verify all DNS records
Use secondary domains for outreach
Avoid too many mailboxes per domain
Sending test emails between similar domains can trigger phishing protection.
Example
If your domain is companyhq.com, avoid testing with:
Fix
Use unrelated domains for testing.
High bounce rates damage sender reputation quickly. A healthy bounce rate should generally stay below 3%.
Fix
Validate contacts before sending
Remove invalid leads regularly
Avoid outdated databases
Some companies use aggressive internal spam filters. This is common with enterprise companies, government organizations and financial institutions.
Fix
Simplify content
Reduce links
Avoid attachments
Use plain-text emails
Login directly into the mailbox and send emails manually. If manual emails land in inbox but campaign emails go to spam, the issue may be related to:
Email copy
Tracking links
Sending platform settings
Sending IPs
Test your sequence by removing elements one by one. For example:
Remove links
Remove signature
Remove images
Simplify wording
Reduce formatting
This helps identify the exact trigger.
Recommended tools: Mail Tester, Mailmeteor Spam Checker and MXToolbox Blacklists. These tools help identify:
Blacklisted domains
Missing DNS records
Spam-triggering copy
Reputation problems
Infrastructure
Use secondary domains
Configure SPF, DKIM, and DMARC correctly
Limit mailboxes per domain
Sending behavior
Warm up new mailboxes for 2 weeks before sending cold emails
Warm up continuously, even after you start cold sending
Send up to 30 cold emails per day per mailbox, and scale slowly
Avoid volume spikes
Keep bounce rates low
Email copy
Personalize emails
Use spintax
Keep emails short
Reduce links
Lead quality
Validate contacts
Target relevant prospects
Avoid outdated lead lists
Pre-warmed mailboxes complete the initial warm-up process, but inbox placement can still be affected by:
Email copy
Links
Sending behavior
Lead quality
Recipient spam filters
If warm-up emails land in inbox but campaign emails go to spam, the issue is usually related to campaign content or targeting rather than mailbox infrastructure itself.
Spam placement is rarely caused by a single issue. Deliverability depends on the combination of:
Healthy infrastructure
Proper warm-up
Good sending behavior
High-quality lead lists
Human-sounding email copy
The best approach is to test incrementally and optimize one variable at a time. Small improvements in copy, targeting, and sending habits can significantly improve inbox placement over time.
