Cold email is one of the most powerful tools for generating leads, especially in B2B environments. But when your messages don’t even make it to the inbox, you’re not just wasting time you’re potentially damaging your brand.

The biggest challenge cold emailers face today is having their email marked as spam. This happens due to a mix of poor technical setup, weak content, and misaligned sending behavior. This guide breaks down the technical reasons for poor deliverability and shows you how to fix them.

 

Why Spam Filters Exist (And How They Work)

Spam filters are complex systems that use hundreds of signals to determine whether a message is safe or suspicious. They consider:

• Sender identity and authentication
• IP and domain reputation
• Message content
• Historical user engagement
• Spam complaint history
• List hygiene and bounce rates

If your campaign fails in any of these areas, the result is often the same your email is marked as spam, and your open rates suffer.

 

Step 1: Set Up Email Authentication

If you send emails from a domain that isn’t properly authenticated, most providers will reject or block your message. Email authentication is the first and most important technical foundation.

Required records:

• SPF (Sender Policy Framework): Tells servers which IP addresses can send on behalf of your domain.
• DKIM (DomainKeys Identified Mail): Encrypts your emails with a signature to verify the content hasn’t been altered.
• DMARC (Domain-based Message Authentication, Reporting & Conformance): Provides instructions for handling suspicious messages and offers insight through reporting.

How to set it up:
Use your domain registrar or hosting provider to add SPF, DKIM, and DMARC DNS records. Tools like Boost Inbox help you configure and validate these settings automatically.

 

Step 2: Warm Up Your Domain and Inbox

Sending a high volume of emails from a brand-new domain or inbox is a major red flag. ISPs monitor email activity and expect gradual growth in sending patterns.

What to do:

• Begin with a small number of emails per day (10-20)
• Gradually increase volume over several weeks
• Encourage replies early to show engagement

Domain warm-up tools can automate this process by sending low-risk messages to trusted inboxes, simulating natural behavior.

 

Step 3: Monitor Sending Behavior

Consistent volume spikes, identical messages to large lists, and mass sending from a single IP will get your email flagged.

Tips for healthy sending patterns:

• Randomize sending times throughout the day
• Rotate multiple sending addresses across campaigns
• Use custom sending domains instead of shared Gmail or Outlook addresses
• Limit each inbox to 30–50 emails/day during warm-up

Platforms like Boost Inbox automatically manage volume and behavior to stay within safe thresholds.

 

Step 4: Maintain List Hygiene

If you send emails to invalid, inactive, or outdated contacts, your bounce rate increases and so does the likelihood of getting flagged as a spammer.

How to clean your list:

• Use verification tools before uploading leads
• Remove hard bounces immediately
• Track unsubscribes and stop emailing disengaged users

Avoid purchasing lists. Build or enrich your own using verified sources.

 

Step 5: Write Emails That Look Natural

Email content still matters. Overuse of promotional phrases, too many links, and generic messaging all increase your spam risk.

Best practices:

• Use plain text with minimal HTML
• Avoid images and attachments in initial cold emails
• Stick to 1-2 links (preferably with full URLs, not shortened)
• Keep it short and relevant (under 150 words is ideal for first touch)

Keep your message structure human. Write like you’re speaking to one person because you are.

 

Step 6: Track Engagement Metrics

Engagement data impacts your sender reputation. If recipients open, reply, or forward your email, that’s a strong positive signal. If they delete it immediately or flag it as spam, your future emails suffer.

What to measure:

• Open rates
• Reply rates
• Spam complaint rate
• Bounce rate
• Unsubscribe rate

Focus on building campaigns that drive actual replies, not just clicks or opens.

 

Step 7: Test Before You Send

Before launching a campaign, test your email’s spam score and inbox placement. Catch problems before they impact your sender score.

Tools to use:

• Mail-Tester
• GlockApps
• Boost Inbox’s built-in spam check

This gives you early warning about issues with your subject line, domain setup, or email content.

 

Bonus: Choose the Right Cold Email Platform

Sending cold outreach from your personal Gmail or a free Mailchimp account isn’t ideal. These services weren’t built for this purpose and lack essential tools for outreach.

Choose a platform that offers:

• Automated domain warm-up
• Built-in deliverability monitoring
• Custom sending schedules
• Integration with CRM or lead enrichment tools

Platforms like Boost Inbox are optimized for cold email and help prevent technical missteps.

 

Conclusion

Cold email isn’t dead bad cold email is. When your email is marked as spam, it’s rarely about the message alone. Most of the time, it’s about how you’re sending it.

Master the technical setup, use clean lists, write like a human, and test thoroughly. These steps don’t just help with deliverability they boost your overall outreach success.