The cold email deliverability playbook

Cold email deliverability comes down to a handful of things you control. Get them right and your mail lands; get them wrong and you sit in spam no matter how good the copy is. This is the practical playbook — no fluff.

We’ll go in the order that matters: authentication, the sending domain, list quality, and volume. Each one compounds on the others.

1. Authenticate: SPF, DKIM, DMARC

SPF says which servers may send for your domain. DKIM cryptographically signs your mail so the recipient can verify it wasn’t tampered with. DMARC ties them together and tells inbox providers what to do if a message fails. Since 2024, Gmail and Yahoo require DMARC for bulk senders — without it you’re throttled or rejected.

Publish all three, and check them with a tool before you send a single campaign. Broken authentication is the single most common reason good mail lands in spam.

2. Use a separate, warmed sending domain

Never cold-email from your primary domain — one spam-trap hit can burn the domain your whole company relies on. Use a dedicated sending domain (often a close variant), and warm it gradually before real volume (see the domain-warmup guide).

Keep marketing and cold-outreach on different domains so a deliverability problem in one never contaminates the other.

3. Clean the list before you send — this is where you win or lose

A high bounce rate is the fastest way to wreck a new domain. Inbox providers read bounces as "this sender doesn’t know who they’re emailing" and start filtering. Under 2% bounce is safe, 2–5% is risky, and above 5% is dangerous.

So verify before you send — and verify properly. The trap is Microsoft 365: SMTP-based verifiers guess on M365 and mark bad addresses "valid", which is exactly where the surprise bounces come from. A verifier that doesn’t guess on M365 — one that keeps your bounce low on those addresses — is what keeps your rate under control on US B2B lists.

4. Ramp volume slowly and watch the signals

Even on a warmed domain, don’t jump to hundreds of sends a day. Ramp gradually, keep your bounce and complaint rates low, and pull back the moment either climbs. Reputation is earned slowly and lost fast.

Personalise enough to earn replies — positive engagement is a deliverability signal too. The goal isn’t volume, it’s landing in the inbox of people who might actually respond.

Takeaway

Authenticate, sail on a warmed dedicated domain, verify your list properly (Microsoft 365 included), and ramp slowly. The single highest-leverage move most senders skip is real verification — that’s what keeps bounces under 2% and your domain healthy.