My Cold Leads
ES
My Cold Leads
Technical Guide

The Complete Guide to Cold Email Deliverability

How to maximize cold email deliverability through technical setup, validation, and copywriting. Everything we do at My Cold Leads to keep bounce rates below 1%.

Jack Kelly Jack Kelly - February 28, 2026

From Jack:

I manage my clients' sender reputations every month so they never land in spam - no matter how many emails we send. I spent weeks putting together every best practice we follow in this 6,000+ word guide so you can hit that <1% bounce rate you're after.

If you have questions, reach out on LinkedIn - happy to help.

Contents

  1. Technical Infrastructure - Domains, mailboxes, authentication, warm-up
  2. Email Validation - Verification, enrichment, catch-all
  3. Smartlead Configuration - Spintax, HTML, SmartServer, protection
  4. Copywriting and Bounces - Copy optimization, case study
  5. Tools List - Every tool we use

1. Technical Infrastructure

Most people already know this: if you send more than 50 emails from your main Google or Office inbox, or try to blast your entire CRM, less than 40% of your emails will land. And if you're using your primary inbox, you'll end up on blacklists.

So the first thing we set up is a technical infrastructure of secondary domains and mailboxes to send small daily volumes per account - but at scale across many accounts.

1.1 Secondary Domains

  • Secondary domains protect your primary domain from reputation damage.
  • They spread sending volume to avoid blocks from email providers.
  • Use 2 mailboxes per secondary domain.
  • Cap at 20 to 30 emails per day per mailbox, permanently.

Example calculation

If I want to send a sequence of 6,000 emails to 2,000 contacts (3 emails each) in one month (20-23 business days):

  • 5 domains
  • 10 mailboxes
  • 10 mailboxes x 30 emails per day = 300 emails per day
  • 300 emails x 20 business days = 6,000 emails = 2,000 contacts

1.2 Mailbox Types

Scale Recommendation Why
1-10 domains Google for Work Simplest setup and best deliverability at low scale
10-25 domains Google Workspace + Office 365 Match mailbox providers with recipients (Google to Google, O365 to O365)
25+ domains 40% Google + 40% O365 + 20% SMTP Diversify risk. SMTP: Mission Inbox for a dedicated IP

1.3 Authentication Setup

We use Zapmail.ai to automate these configurations.

  • Set up SPF, DKIM, DMARC, and MX records correctly for each domain
  • Each domain is assigned to a separate Google Workspace or Office 365 account
  • We implement domain masking redirects (anyone looking up the sending domain gets redirected to your main website)
  • Add profile photos to sender accounts

Verification tools: MxToolbox, EasyDMARC

1.4 Warm-up Process

  • Warm up mailboxes for at least 2 weeks (Google) or 4 weeks (Office)
  • Start with 1-5 emails per day, gradually ramping up to 30 per day
  • Vary daily sending volumes (1, 4, 3, 7, 6, 8, 14, 12, etc.) to look organic
  • Simulated reply rate of 30-40%
  • Tool: Smartlead.ai (integrates with Zapmail.ai)

Section 1 Summary

  1. Buy secondary domains (1 per 2 mailboxes)
  2. Create 2 mailboxes per domain
  3. Choose server type based on scale
  4. Configure SPF, DKIM, DMARC, and MX records
  5. Enable domain masking redirects
  6. Add profile photos to mailboxes
  7. Automate setup with Zapmail
  8. Run warm-up (min. 2 weeks Google, 4+ weeks Office)

2. Email Validation

2.1 Email Verification

  • Use MillionVerifier (best value for money) to validate emails
  • Upload a CSV, get back the verified ones
  • Remove invalid addresses
  • Catch-all: optional (bounceban.com can validate them), but if you want to cut bounces further, drop them
  • MillionVerifier has a direct integration with Smartlead

2.2 Process with Clay

  1. Extract contacts from your data source
  2. Use Clay to find and enrich contacts with emails (waterfall enrichment)
  3. Run through MillionVerifier, Zerobounce, or Debounce to validate and clean
  4. Only import verified emails into campaigns

Section 2 Summary

  1. Extract contacts with Clay (or another tool)
  2. Validate emails with MillionVerifier, ZeroBounce, or Debounce
  3. Remove invalid and catch-all addresses (optional depending on strategy)
  4. Push verified emails directly to Smartlead or another sending platform
  5. Import clean list into your campaign

3. Smartlead Configuration

3.1 Contact Import

  • Smartlead has a massive database of emails that have bounced before
  • Enable the option to automatically remove any recent bounces from your campaign
  • Option to verify emails with Smartlead as an extra layer

3.2 Writing with Liquid Spintax

To dodge spam filters, use Liquid Spintax to create variations:

Example: {Hey|Hi|What's up} John,

Result: "Hey John", "Hi John", or "What's up John" at random.

Pro tip: Give ChatGPT your email template and ask it to generate a version with Liquid Spintax built in.

Key tips:

  • Avoid spammy words and structures
  • Keep messages simple - no links or images
  • If the prospect wants to check your website, they will (domain masking via Zapmail handles this)
  • AI can personalize messages for lists of thousands of recipients using tools like Clay

3.3 Plain HTML Format vs. Open Tracking

  • Tracking pixels can add negative points in anti-spam filters
  • Stop tracking opens - open rates are increasingly unreliable (Gmail preloads images)
  • Sending in pure HTML format (no pixels) can improve deliverability
  • Prioritize delivery over inaccurate metrics

Note: If you want to send images, you won't be able to enable these options.

3.4 SmartServer and Provider Matching

  • Assign a SmartServer if you're sending more than 1,000 emails/day. It gives you a dedicated IP exclusively for your sends.
  • Enable mailbox provider matching if you've set up mailboxes on different providers (Google & O365).

3.5 Bounce Protection

Enable auto-protection against high bounce rates. Recommended threshold: 2-3%.

3.6 Mailbox Monitoring

  • Check mailbox health every single day after launching a campaign
  • If warmup reputation drops, pull the mailbox from campaigns for a few days and let it warm up until it recovers to 100%
  • Buy additional domains/mailboxes for rotation

Section 3 Summary

  1. Enable shared bounce removal (automatic cleaning) via Smartlead
  2. Turn on Smartlead verification if needed
  3. Write emails with Liquid Spintax (use GPT to automate variations)
  4. Avoid links, images, and spammy words in your copy
  5. Use pure HTML - no tracking pixels
  6. Enable SmartServer if sending >1,000 emails/day
  7. Match mailboxes by provider (GfW to GfW, O365 to O365)
  8. Turn on bounce protection (>2-3% cutoff)
  9. Monitor mailbox reputation/warmup daily

4. Copywriting and Bounces

4.1 Continuous Message Improvement

  • Write your copy and run it through AI (Claude recommended)
  • Ask whether it contains words that could trigger spam filters
  • Smartlead offers copy deliverability analysis (paid) - alternative: GlockApps

4.2 Analyzing Copy Bounces in Smartlead

Smartlead shows whether bounces are caused by list quality or copy. If copy bounces exceed 2-3%, stop and troubleshoot.

4.3 Case Study

Before (high bounce)

Felipe, I was on your website and it looks like you sell to software leaders. It seems like you're trying to help them grow and transform through GAI implementation.

I assumed you're using cold prospecting tactics since Luis Garcia is a Business Development Director on your team.

Are you thinking about how to leverage better data and GPT-4 for prospecting?

I ask because we help companies automate their cold email outreach with large databases, GPT-4, and automated sending campaigns.

Would you be open to a chat about building a prospecting process tailored to you?

After (optimized)

Pablo, I checked out your site and noticed you focus on food industry leaders. Looks like you help them measure and report on food sustainability efficiently.

I assumed you're running cold outreach since Sebastian Arias is a Sales Development Manager on your team.

Are you thinking about how to use better data and AI for contact research?

I ask because we help companies streamline conversations with potential clients using public data, AI, and the right tools.

Would it make sense to hop on a call about setting up a personalized lead generation system for you?

Section 4 Summary

  1. Review copy with AI to catch problematic words
  2. Use Smartlead's copy bounce analysis feature
  3. Adjust text if bounce rate exceeds 3%
  4. Test copy versions before scaling
  5. Document learnings and keep iterating

5. Tools and Resources

Technical Setup and Mailboxes

Zapmail.ai - Automates technical setup (SPF, DKIM, DMARC, MX) and domains

Google Workspace - Scalable mailbox creation

Office 365 - Alternative for mixing providers

Mission Inbox - Enterprise SMTP for high-volume sending

MxToolbox - DNS and email record verification

EasyDMARC - Monitors SPF, DKIM, DMARC setup and reputation

Email Validation

MillionVerifier - Bulk email validation (our top pick for quality and value)

ZeroBounce / Debounce - Reliable alternatives for email cleaning

BounceBan.com - Additional catch-all validation

NeverBounce - Another solid option with multiple layers

Lead Extraction and Enrichment

Clay.com - Extraction, enrichment, and validation from a single interface

LeadMagic - Lightweight tool for contact enrichment

Email Warming and Campaign Sending

Smartlead.ai - Sends cold emails, manages domains/mailboxes, automates warm-up

Instantly.ai - Versatile alternative for cold outreach with built-in warm-up

Lemwarm (Lemlist) - Complement if you already use Lemlist

Copywriting and Testing

ChatGPT / Claude - For writing copy and building Liquid Spintax

Smartlead Copy Analyzer - Detects deliverability issues in your text

GlockApps.com - Spam testing, inbox placement, and reputation

Monitoring and Inbox Placement

Postmaster Tools (Google) - Native monitoring for GfW domains

Smartlead Reputation Dashboard - Real-time mailbox health and impact assessment

Mail-Tester.com - Quick, free pre-send testing tool (three tests per day only)

Other Useful Resources

Notion / Airtable - Organize workflows, lead databases, and projects

Calendly - Automated scheduling for post-outreach calls

Loom - Personal video recordings for cold email with video (optional)

Maildoso - Good but pricey platform if you want to outsource technical infrastructure

Conclusion and Next Steps

And that's it! With this guide you have the foundation of an effective, automated email infrastructure ready to scale your sales.

If you have any questions - reach out. Happy to help.

And if you'd like us to handle it for you, you can book a call with me here.

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