Blog · Email Infrastructure
February 20, 2025 · 8 min read
Your sending domains are the foundation of your cold email operation. Get them right and every other part of outbound becomes easier. Get them wrong and you will spend months troubleshooting deliverability problems that should never have existed.
The first principle: never send cold email from your primary domain. If your company is acmecorp.com, your cold email should not come from @acmecorp.com. It should come from a dedicated sending domain — something like acme-hq.com or hireacme.com or acmesales.io. If that domain's reputation takes a hit, your primary domain is protected.
Buy two to four domains per campaign. Rotate sending across all of them to spread the load. Choose domains that look legitimate — not spammy. Variations on your main brand, industry terms combined with your brand name, or TLDs like .io or .co all work. Avoid generic-sounding domains that look like they were spun up to spam people, because spam filters will make the same assessment.
Purchase domains from a registrar and immediately connect them to a Google Workspace or Microsoft 365 account. You need actual mailboxes, not forwarding addresses. Start with two mailboxes per domain — you can scale up later.
Before your first email goes out, three DNS records need to be configured: SPF, DKIM, and DMARC. SPF (Sender Policy Framework) tells receiving mail servers which servers are authorised to send email on behalf of your domain. DKIM (DomainKeys Identified Mail) adds a cryptographic signature to every email, proving it has not been tampered with. DMARC (Domain-based Message Authentication, Reporting and Conformance) tells receiving servers what to do if SPF or DKIM checks fail.
Your email provider — Google or Microsoft — will give you the DNS records to add to your registrar. The process takes ten minutes. Skipping it is not an option: sending without SPF and DKIM is the fastest way to get flagged as spam.
A brand new domain has no sender reputation. Start sending from it immediately and most of your emails will land in spam. The domain needs to be warmed up — gradually increasing send volume while maintaining high engagement — before campaign sending begins. Most teams use a dedicated warmup tool like Instantly's warmup network or Mailreach for this.
Warmup typically takes three to four weeks. After that, start campaign sends at a conservative volume — twenty to thirty emails per inbox per day — and scale up slowly. Watch your deliverability data. If open rates drop or spam complaints appear, pull back before the damage compounds.