Ultimate Guide to Cold Email Deliverability
Everything you need to land in the inbox, avoid spam folders, and protect your sender reputation.
BookingBomb Team
January 8, 2025
The Problem
21% of legitimate emails never reach the inbox.
Your perfectly crafted cold email means nothing if it lands in spam. Here's how to fix that.
You wrote the perfect email. Compelling subject line. Personalized opening. Clear value prop. And it went straight to spam.
Deliverability is the invisible killer of cold email campaigns. Companies spend thousands on copywriters and data, then watch their emails disappear into the void because they skipped the technical foundation.
This guide covers everything you need to know about cold email deliverability—from DNS records to warmup strategies to ongoing monitoring.
What We'll Cover:
- • The three pillars of email authentication (SPF, DKIM, DMARC)
- • Domain and mailbox setup best practices
- • Email warmup: why it matters and how to do it
- • Content that triggers spam filters
- • Monitoring and maintaining sender reputation
- • Recovery: what to do when you're already in spam
Email Authentication: SPF, DKIM, and DMARC
These three acronyms determine whether email providers trust your messages. Get them wrong, and you're flagged as a potential spammer before your email is even read.
SPF (Sender Policy Framework)
SPF tells email servers which IP addresses are allowed to send email on behalf of your domain.
Example SPF Record:
v=spf1 include:_spf.google.com include:sendgrid.net ~allDKIM (DomainKeys Identified Mail)
DKIM adds a digital signature to your emails, proving they haven't been tampered with in transit.
How DKIM Works:
DMARC (Domain-based Message Authentication)
DMARC ties SPF and DKIM together and tells receiving servers what to do with emails that fail authentication.
Recommended DMARC Record for Cold Email:
v=DMARC1; p=none; rua=mailto:dmarc@yourdomain.com; pct=100p=none: Monitor mode—don't reject emails yet
rua: Email address for aggregate reports
pct=100: Apply policy to 100% of emails
Common Authentication Mistakes
- • Multiple SPF records (you can only have one)
- • Forgetting to add sending platforms to SPF
- • Using p=reject on DMARC before monitoring
- • Not updating DNS after changing email providers
Domain and Mailbox Setup
Never send cold email from your primary domain. Ever. Here's the proper infrastructure setup.
Use Separate Domains for Cold Outreach
Domain Strategy:
Primary domain (company.com)
Website, inbound email, important communications only
Cold outreach domains (getcompany.com, trycompany.com, hellocompany.com)
Dedicated to cold email—protects your main domain if issues arise
Mailbox Math
3-5
Domains per campaign
2-3
Mailboxes per domain
30-50
Emails per mailbox/day
200-500
Total daily sending capacity
Email Warmup: The Critical First Step
A new mailbox with no sending history is suspicious to email providers. Warmup builds trust before you start your campaigns.
How Email Warmup Works
Automated sending
Warmup tools send emails between real mailboxes in their network
Positive signals
Emails are opened, replied to, and moved out of spam
Gradual increase
Volume ramps from 5-10 emails/day to 30-50 over 2-4 weeks
Reputation building
Email providers see positive engagement and trust your domain
| Week | Warmup Emails | Cold Emails | Total |
|---|---|---|---|
| Week 1-2 | 10-20/day | 0 | 10-20 |
| Week 3 | 20-30/day | 5-10/day | 25-40 |
| Week 4 | 20-30/day | 15-25/day | 35-55 |
| Week 5+ | 15-20/day | 30-50/day | 45-70 |
Content That Triggers Spam Filters
Even with perfect technical setup, your content can still land you in spam. Here's what to avoid.
Spam Trigger Words
Avoid: "Free," "Act now," "Limited time," "Guarantee," "No obligation," "Click here," "Buy now," "$$$." These are instant red flags.
Too Many Links
One link maximum in cold emails. Multiple links = marketing email = spam filter trigger. Save the links for follow-ups.
Images in First Email
No images, logos, or signatures with images in your first cold email. Plain text performs best and avoids filters.
Link Shorteners
Never use bit.ly, tinyurl, or similar shorteners. Spammers use them to hide malicious links. Use full URLs or custom tracking domains.
ALL CAPS or Excessive Punctuation!!!
Looks spammy, triggers filters, and annoys recipients. Write like a professional human.
Content Best Practices
- • Keep emails under 125 words for first touch
- • Use plain text formatting (no HTML)
- • Personalize the first line with research
- • Include your real name and company
- • Add an unsubscribe option (legally required)
Monitoring Your Sender Reputation
Deliverability isn't set-and-forget. You need to continuously monitor and adjust.
Key Metrics to Track
Recovery: When You're Already in Spam
If your deliverability has tanked, here's the recovery playbook:
Stop All Cold Outreach Immediately
Every email you send while your reputation is damaged makes it worse. Pause everything.
Audit Your Technical Setup
Re-verify SPF, DKIM, and DMARC. Check for blacklist listings. Fix any issues found.
Aggressive Warmup (2-4 Weeks)
Use warmup tools at maximum settings. Focus on positive engagement signals. No cold email during this period.
Restart Slowly
Begin with 10-15 emails per day. Monitor metrics closely. Scale up only when numbers stabilize.
If Recovery Fails: Fresh Start
Sometimes domains are too damaged. Register new outreach domains, set them up properly, and start fresh with proper warmup.
The Bottom Line
Deliverability is the foundation of cold email success. Without it, nothing else matters. The good news: it's not complicated. It just requires doing the boring technical work upfront.
Your Deliverability Checklist
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