2026 Primary-Inbox Architecture • ≤ 35 Cold Sends / Mailbox / Day

Cold Email Deliverability & Secondary Domain Warmup Checklist

Even the highest-intent Seed, Series A, and hiring signals fail if your emails land in spam. Calculate your required secondary sending domains and mailbox count below, and review our 4-pillar primary-inbox checklist.

1. Calculate Required Sending Mailboxes & Secondary Domains

Current Load Per Mailbox
38 sends/day

Moderate Promotions-Tab Risk (36–50 sends/inbox/day)

Recommended Safe Mailboxes
5 Inboxes

Capped at ~30 cold prospects/mailbox/day

Secondary Sending Domains
3 Domains

2 mailboxes per isolated secondary domain

2. Filter 2026 Deliverability & Warmup Controls (1 Shown)

Verify DNS authentication, 14-day warmup, plain-text copy, and send throttling:

All Controls (4) SPF / DKIM / DMARC 14-Day Warmup Plain-Text Copy Send Throttling
1. DNS Authentication & Secondary Domains

Isolate Cold Outbound on Dedicated Secondary Domains with Strict SPF, DKIM & DMARC

Never send cold outbound from your primary corporate domain. Provision 2 mailboxes per secondary domain (e.g., get-agency.com, try-agency.com) with 2048-bit DKIM keys and a 301 root redirect to your main site.

SPF: v=spf1 include:_spf.google.com ~all DMARC: v=DMARC1; p=quarantine; rua=mailto:dmarc@your-domain.com; pct=100 DKIM: 2048-bit RSA selector enabled in Google Workspace / Microsoft 365
Deliverability Impact: Protects your primary domain's transactional and client calendar reputation while meeting 2026 Google/Yahoo bulk-sender mandates.

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⬇ Download Free Sample CSV View Clay & Instantly Sequence Playbook → Custom Outbound Infrastructure Scoping →