Nobody notices deliverability until it's the reason a campaign is failing. The copy can be perfect, the targeting flawless, the personalization genuine — and it won't matter if half your sends never reach an inbox. Protecting your sending domain is unglamorous, entirely technical, and the single most common reason first-time outbound campaigns underperform.
SPF, DKIM, and DMARC — the non-negotiables
These three DNS records are how mailbox providers verify that an email claiming to be from your domain actually is. Without them, you're not just risking spam-folder placement — you're leaving your domain open to being spoofed by anyone.
- SPF (Sender Policy Framework) lists which mail servers are authorized to send on behalf of your domain.
- DKIM (DomainKeys Identified Mail) attaches a cryptographic signature to outgoing mail, proving it wasn't altered in transit.
- DMARC tells receiving servers what to do when SPF or DKIM checks fail — and gives you visibility into who's sending mail as your domain.
All three should be in place, correctly configured, before a single cold email goes out — not retrofitted after deliverability tanks.
Use a separate sending subdomain
Never run cold outreach from your primary domain. If company.com handles invoices, support tickets, and internal mail, a deliverability issue on a cold campaign can drag your entire domain's sender reputation down with it. Sending from a dedicated subdomain — something like outreach.company.com — isolates that risk while still inheriting enough domain trust to build reputation faster than a brand-new domain would.
Warm up before you scale
A new sending domain or subdomain has no reputation yet, and mailbox providers treat sudden high-volume sending from an unknown source as a spam signal by default. Warm-up means starting at low daily volume and gradually increasing over several weeks, allowing providers to build a positive reputation profile for your domain based on genuine engagement — opens, replies, low bounce and complaint rates.
Deliverability is a reputation you build slowly and can lose in a single bad week of sending.
List hygiene isn't optional
High bounce rates are one of the fastest ways to damage sender reputation. Verify email addresses before sending, remove hard bounces immediately, and never re-import a list that previously generated spam complaints without re-verifying it. A smaller, clean list will always outperform a larger, unverified one — both in deliverability and in the numbers that actually matter downstream.
Monitor, don't assume
- Track bounce rate — keep it under 2-3%.
- Watch spam complaint rate — even a small spike can trigger provider-level throttling.
- Check blocklist status regularly for your sending domain and IPs.
- Segment sending volume across multiple mailboxes rather than pushing everything through one address.
None of this replaces good targeting or good copy — but it does determine whether that copy ever gets read at all. Once your infrastructure is solid, the next lever is sequencing across channels: see the multi-channel playbook, or the full rollout timeline in the 90-day framework.