The Email Deliverability & Infrastructure Playbook

This is the infrastructure play we run for every client before a single cold email goes out: secondary domains, full DNS authentication, a two-week warmup, and hard volume caps per mailbox. Inbox placement is the foundation everything else sits on. You can have the best list, the sharpest copy, and the best offer in your industry, and still fail completely if your emails land in spam.

Why do most cold emails land in spam?

Roughly 95% of cold emails never reach the inbox. Gmail and Outlook AI filters have grown teeth, and the bar keeps rising. Spam complaints from one bad campaign can burn a domain for weeks. Send from your primary domain and you risk the whole company's email reputation.

Done right, deliverability is invisible. Done wrong, it kills every play that depends on it.

How should you structure sending domains?

We never send cold email from your primary domain. Ever. Instead we buy 5-15 secondary domains specifically for outbound, each closely matching your brand (yourbrand.io, yourbrand-team.com, get-yourbrand.com). They redirect traffic to your real site, so they look legitimate to filters and to prospects.

• Primary domain stays clean for transactional and customer email
• Secondary domains are disposable: if reputation degrades, we rotate
• Each domain hosts 2-3 mailboxes, never more, to avoid sender reputation pooling

What DNS authentication does cold email need?

Three protocols verify that the sender is who they claim to be. Without all three configured correctly, you are flagged as suspicious from the first send.

• SPF (Sender Policy Framework) lists the mail servers authorised to send on behalf of your domain. Without it, providers cannot verify the message came from you.
• DKIM (DomainKeys Identified Mail) cryptographically signs each message, so recipients can verify it was not tampered with in transit.
• DMARC (Domain-based Message Authentication, Reporting & Conformance) is the policy layer telling providers what to do with messages that fail SPF or DKIM.

We set all three on every secondary domain, with DMARC starting in monitoring mode (p=none) and tightening to quarantine once the domain has reputation.

How do you warm up new mailboxes?

New domains and mailboxes have zero reputation. Send 200 cold emails on day one and the domain gets cooked. We warm every mailbox for at least 2 weeks before any real outreach happens.

Phase	Warmup volume	Goal
Week 1	5 emails per day per mailbox, ramping by 5/day	40%+ reply rate to signal real engagement to providers
Week 2	Ramp to 30-40 emails per day	Mailbox ready for real sends by end of week
Ongoing	10-15 emails per day during real campaigns	Maintain reputation

How much cold email can you send per mailbox?

We cap real outbound at 30-40 emails per mailbox per day. Going higher gets you flagged. Gmail's hard limit is 500/day and Outlook's is 300/day, but those are theoretical maximums, not targets. Real-world sustainable volume is much lower.

Volume scales by adding mailboxes, not by pushing each mailbox harder. 10 mailboxes at 35/day is 350 sends/day with stable reputation. 1 mailbox sending 350/day is a blacklist within a week.

How do you keep sender reputation healthy?

• Bounce rate: keep it under 2%. Always verify every list with Bounceban before sending.
• Spam complaint rate: keep it under 0.1%. One bad list will kill a domain.
• List hygiene: re-verify lists every 3-6 months. Old data goes stale fast.
• Engagement metrics: watch open and reply rates. Drops of 30%+ versus baseline mean reputation is degrading.

What tools do we use?

Spaceship for bulk domain purchasing. Mailin for mailbox setup and warmup at scale. Smartlead, Instantly, EmailBison, or Lemlist for sending. Bounceban for pre-send verification. Mailreach or Glock Apps for inbox placement testing.

What should you watch for?

• Domain age matters. New domains under 30 days old face heavier scrutiny. We buy domains 4-6 weeks before launch where possible.
• Workspace isolation. Never share IP space across multiple domains. Each domain gets its own dedicated workspace.
• Replace domains every 9-12 months. Even with perfect hygiene, reputation eventually decays. Plan for rotation.
• Sender names that look fake trigger filters. Generic-looking senders get flagged faster than real ones.

FAQ

Should you send cold email from your primary domain?

Never. One bad campaign can burn a domain's reputation for weeks, and you cannot risk the whole company's email. We buy 5 to 15 secondary domains that closely match the brand, redirect them to the real site, and keep the primary domain clean for transactional and customer email.

How many mailboxes should each sending domain host?

2 to 3 mailboxes per domain, never more, to avoid sender reputation pooling. Volume scales by adding domains and mailboxes, not by pushing each mailbox harder.

How long does email warmup take?

At least 2 weeks before any real outreach. Week 1 starts at 5 warmup emails per day per mailbox, ramping by 5 per day. Week 2 ramps to 30 to 40 per day. We then keep 10 to 15 warmup emails running daily during real campaigns to maintain reputation.

How many cold emails can one mailbox send per day?

We cap real outbound at 30 to 40 emails per mailbox per day. Gmail's hard limit is 500 and Outlook's is 300, but those are theoretical maximums. 10 mailboxes at 35 per day gives 350 stable sends. One mailbox sending 350 per day gets blacklisted within a week.

What DNS records does cold email require?

All three authentication protocols: SPF, which lists the servers authorised to send for your domain, DKIM, which cryptographically signs each message, and DMARC, the policy layer telling providers what to do with failures. We start DMARC in monitoring mode and tighten to quarantine once the domain has reputation.

How often should sending domains be replaced?

Every 9 to 12 months. Even with perfect hygiene, domain reputation eventually decays, so we treat secondary domains as disposable and plan for rotation from day one.