How the calculator works
First, prospects are multiplied by the number of planned email touches. The calculator then divides those monthly sends by the usable capacity of one mailbox. Usable capacity is your daily rate multiplied by send days, with the reserve removed. Mailboxes are rounded up because a fraction of an inbox cannot carry a campaign.
The result is shown as a range. The lower infrastructure count uses your upper daily sending assumption; the higher count uses the conservative assumption. We would normally plan from the conservative end and treat the smaller number as an optimistic boundary, not a target to squeeze into. The three-to-four-week readiness reference is separate operating guidance; it does not change the capacity calculation.
Why hold capacity in reserve?
A spreadsheet assumes every mailbox sends every planned day. Real operations include provider pauses, replacement inboxes, campaign variation and accounts that need to rest. Reserve prevents the plan from depending on perfect utilization. If the motion is new, start with more reserve and revise from observed performance.
Why use secondary domains?
The primary company domain carries day-to-day customer, partner and internal communication. Cold outreach creates a different reputation profile, so the sending layer should be isolated on clearly related secondary domains. Authentication and warm-up still matter; isolation simply keeps experimentation away from the domain the business cannot afford to lose. For infrastructure-provider trade-offs, see Mailpool vs Mailforge and Mailpool vs Infraforge.
From capacity to a real motion
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