How many mailboxes and domains do I need?
Cold email is sent from many mailboxes, spread over several domains, so that no single mailbox or domain carries the whole volume. How many you need follows from four things: how many leads you email, how many emails each lead gets in the sequence, how many emails one mailbox sends a day, and how many days you give the campaign. How many mailboxes sit on one domain then sets the number of domains.
Reading the result
The calculator multiplies your leads by the emails in the sequence to get the total, shares that across your sending days, and divides the emails a day by what one mailbox sends, rounding up. The mailboxes divided by how many you put on a domain, rounded up, is the number of domains. Every step is written out under the result, and the drawing shows each domain with its mailboxes.
What the numbers leave out
Warm-up time
The result counts mailboxes at full volume. A new mailbox on a new domain is usually brought up gradually, so the first weeks send less; the domain and inbox planner works out how long a ramp takes.
Follow-ups landing after the window
Every email of the sequence is counted inside your sending days. If the last follow-ups go out after the last day, the busiest days come in the middle of the campaign, and a few extra mailboxes give that room.
Treating the per-mailbox number as a provider limit
Google and Microsoft publish the most an account may send before they stop it, which is far above what cold email senders choose. The number per mailbox is your own judgement, and the domain and inbox planner sets it against the providers' published limits.
Replies and bounces
Leads who reply usually leave the sequence, and addresses that bounce get no follow-up, so the real total is a little lower than leads times steps. The calculator counts the full sequence for everyone, which leaves a margin.