The number of sending domains you need for a cold email campaign is determined by your target daily sending volume, the number of mailboxes per domain, and the number of emails each mailbox sends per day. The math is straightforward but the answer surprises most people who are new to cold email infrastructure.
The Math Behind Domain Count
Each sending domain should host 2 to 3 mailboxes. Each warmed mailbox sends 30 to 50 emails per day. So each domain produces 60 to 150 emails per day at safe sending rates. A campaign targeting 500 emails per day needs 4 to 8 domains. A campaign at 1,000 emails per day needs 8 to 15 domains. These numbers are calibrated for sustainability — exceeding 3 mailboxes per domain or 50 emails per mailbox per day starts damaging domain reputation measurably over 60 to 90 day time horizons.
Why You Should Never Send Cold Email From Your Primary Domain
Your primary business domain — the one on your company website and all business communication — should never send cold email. If the cold email domain gets blacklisted or damages its reputation, it only affects that domain. Your primary domain stays clean for critical business emails, vendor communication, and inbound replies. Dedicated cold email domains that closely resemble your primary domain (yourdomain-sales.com, getyourdomain.com, yourdomain.io) are the professional standard.
Scaling Domain Count Over Time
New domains need 3 to 4 weeks of warmup before they can send cold email. This means scaling a campaign takes time — you cannot decide to double volume this week. A 30-day runway for domain registration and warmup is needed before any volume increase. Omni registers and warms domains in anticipation of campaign growth needs, maintaining a buffer of ready-to-use infrastructure for client campaigns. Domain infrastructure management is covered in the outbound system overview.