The number of mailboxes you need for a cold email campaign is a function of your target daily sending volume, the safe sending limit per mailbox, and how many mailboxes you want to run per domain. Getting this calculation right upfront prevents the most common cold email infrastructure mistake: launching with too few mailboxes, overloading them, and damaging your domain reputation within the first 30 days.
The Safe Sending Limits
A properly warmed cold email mailbox should send no more than 30 to 50 emails per day. This is the range that inbox providers treat as consistent with normal individual sender behavior. Above 50 per day, sending patterns become statistically detectable as bulk sending. Above 100 per day, inbox placement rates decline measurably over 60 to 90 day time horizons even with strong authentication and warmup.
Calculating Mailbox Count
Daily sending target divided by 40 (mid-range safe limit) gives your required mailbox count. A campaign targeting 200 daily sends needs 5 mailboxes. At 500 sends per day: 13 mailboxes. At 1,000 sends per day: 25 mailboxes. Group mailboxes 2 to 3 per domain — so 25 mailboxes requires 8 to 12 dedicated sending domains. All of these need to be warmed 3 to 4 weeks before any cold email is sent.
Planning for Campaign Duration
A domain used for cold email has a useful life of 6 to 18 months depending on how carefully it is managed. Build your infrastructure to support a multi-month campaign, not just the first send. Omni registers domains in advance of campaign needs so warmed infrastructure is always available when clients are ready to scale. Full infrastructure planning is part of the done-for-you outbound system.