The Deliverability Guide

Inbox placement is the foundation. Everything else is wasted effort.

You can spend weeks crafting the perfect ICP, sourcing verified data, and writing signal-led copy — and if your emails land in the spam folder, none of it matters. Deliverability is the silent killer of cold outreach. Most campaigns fail here, and most teams never realise it because mailbox providers don't tell you when they're filtering you out.

The numbers are brutal: across the industry, the average cold email campaign sees 70–80% inbox placement at best. That means up to 3 out of every 10 emails you send are invisible. Worse — once your domain reputation starts to decay, every subsequent campaign gets quieter, until you wonder why nothing works anymore.

SPF, DKIM, DMARC — the non-negotiable foundation.

Email authentication is how mailbox providers verify you're not spoofing someone else's domain. Without it, you'll get filtered to spam regardless of how clean your copy is. There are three records you must configure, in this order:

SPF (Sender Policy Framework)

SPF tells the world which servers are allowed to send email on your behalf. Without it, anyone can spoof your domain.

Common mistake: Using +all instead of ~all or -all. The +all allows any server to send — defeating the purpose entirely.

DKIM (DomainKeys Identified Mail)

DKIM signs every outgoing email cryptographically, proving the message wasn't tampered with in transit. Most sending platforms generate the DKIM key for you — you just paste the TXT record into your DNS.

DMARC (Domain-based Message Authentication)

DMARC tells receivers what to do when SPF or DKIM fail — and gives you reporting so you can see who's trying to spoof your domain.

Start with p=none (monitoring only) for the first 30 days. Once you've confirmed all legitimate senders are aligned, move to p=quarantine, then eventually p=reject.

Dedicated domains. Never your primary.

This is the single most important rule in cold outreach: never send cold email from your primary business domain. If your main domain gets blacklisted, you lose more than campaigns — you lose customer support, billing, contracts, and every other email that matters.

The dedicated domain setup

1. Buy 2–4 sending domains that are close variations of your primary. If your main domain is acme.com, buy acme-team.com, get-acme.com, tryacme.io, acme-hq.com.
2. Set up forwarding from each sending domain to your primary domain — so replies land in your real inbox.
3. Build a simple landing page on each sending domain that redirects to your main site. Cold prospects who click the domain in your signature should see a legitimate brand presence.
4. Configure full authentication — SPF, DKIM, DMARC, and rDNS — on every sending domain independently.

Mailbox setup

Within each domain, set up 3–5 mailboxes — typically using real-sounding names like firstname@domain.com. Use professional providers like Google Workspace or Microsoft 365. Avoid SMTP-only relays for cold outreach — they get flagged faster.

You can't start at 150/day. Warmup or die.

A brand new mailbox sending 150 cold emails on day one is a dead mailbox. Mailbox providers watch for sudden volume spikes — it's the single biggest spam signal. The fix: structured warmup over 3 weeks before any campaign launches.

The 3-week warmup ramp

Mailbox rotation

Inside your sending tool, configure round-robin rotation across all mailboxes. Each mailbox carries its own reputation — spreading volume protects them all. Smartlead, Instantly, and Lemlist all handle this natively. Configure it once and forget it.

The 150/day rule (and when to break it).

The hard ceiling for cold outreach is 150 emails per mailbox per day. Could you technically send more? Yes. Should you? Almost never. The reputation cost compounds — 200/day might give you 30% more volume this week, but in three weeks your domain is in trouble.

The smarter scaling strategy isn't more volume per mailbox — it's more mailboxes at the same volume. Need 5,000 emails/day? That's 34 mailboxes at 150/day, not 10 mailboxes at 500/day.

What you write affects whether it lands.

Mailbox providers run every incoming email through content filters. Even with perfect authentication, certain words, structures, and patterns will route you to spam. The rules aren't always intuitive — but the pattern is consistent: cold email should look like a human one-to-one note, not a marketing blast.

Spam trigger words to avoid

The classic list — "free," "guarantee," "limited time," "act now," "click here" — still applies. But modern filters care more about structure than individual words. A single instance of "free" is fine. Three exclamation marks isn't.

Structural rules

• No images in cold emails. Images load via tracking pixels, which mark you as bulk mail. Use them in nurture sequences only.
• One link maximum — and that link should be to your domain, not a tracker.
• Plain text formatting. No HTML signatures, no colored fonts, no bold/italic. Looks like a real person typed it.
• 50–120 words ideal length. Under 50 looks lazy; over 150 looks like a pitch.
• No tracking pixels. Open tracking inflates open rates but hurts deliverability by 8–12%. Turn it off.

If you're not watching, you're already losing.

Deliverability decays silently. By the time you notice a drop in reply rates, you've already lost weeks of pipeline. The fix is weekly monitoring across three layers — placement, reputation, and blacklists.

The weekly monitoring stack

1. GlockApps inbox placement tests — send a test campaign to a seeded inbox panel covering Gmail, Outlook, Yahoo, and corporate domains. Report shows exactly where you're landing.
2. Google Postmaster Tools — Google's own dashboard showing your domain reputation, spam rate, and authentication failures. Free, and the most reliable signal you can get for Gmail recipients.
3. MXToolbox blacklist scan — checks your IPs and domains against 50+ public blacklists. Even one listing can tank delivery.
4. Daily bounce rate review — bounces above 3% mean your data quality is failing. Pause and re-verify your list before continuing.

When deliverability tanks — the recovery playbook.

It happens. A bad list, a spam-trap, or a sudden volume spike — and suddenly you're in spam everywhere. Don't panic, don't keep sending. The recovery process takes 2–4 weeks of disciplined work, not a vendor switch.

The 4-step recovery

1. Stop all sending immediately. Every additional bad send digs the hole deeper. Pause for 48 hours minimum.
2. Audit the damage. Run inbox placement tests, check blacklists, review Google Postmaster. Identify exactly where you're being filtered and why.
3. Fix the root cause. If it was data quality, re-verify your list. If it was volume, reduce by 50%. If it was content, rewrite from scratch.
4. Restart with extreme caution. Drop to 10 emails/day per mailbox for week one. Slowly ramp back over 3 weeks — same as initial warmup. Treat the mailboxes as if they're brand new.

That's the entire deliverability system in one document. Authentication, infrastructure, warmup, volume, content, monitoring, recovery — run all seven and you'll consistently hit 96%+. Skip any one, and you'll wonder why your outreach doesn't work anymore.