Email unit
An email unit is the usage unit for agent mail, counting one message received or one recipient sent to, so a single send addressed to three people consumes three units and test mail consumes none.
“Emails per month” is an ambiguous unit and most pricing pages leave it that way. An email unit resolves the ambiguity in one direction: it counts recipients on the way out and messages on the way in, so the number on your usage page is the number of times the system did work on your behalf.
The two meters
Usage is written to a ledger under two meter names, and knowing which one fired explains every number you will see.
inbound_delivery is debited once per message received, whatever the message
contains and however many addresses were on it. A message sent to your agent and
copied to three colleagues is one inbound unit for you.
outbound_recipient is debited once per unique recipient address, at the moment the
message state becomes accepted. A send to three people is three debits; a send to
the same address in both To and Cc is one, because the meter counts unique addresses.
When the debit happens, and when it comes back
Acceptance is the commit point. Not queuing, and not delivery.
Queuing is too early because a message can still be refused by the provider. Delivery is too late because delivery is reported asynchronously by a third party and may never be reported at all. Acceptance is the last event anyone on this side can directly observe, so that is where billing lands. The difference between those three states is the subject of what an email status actually proves, and it is worth reading before you reconcile a usage figure against your own send log.
There is one reversal. When a provider reports that an address is suppressed, the
debited outbound_recipient unit for that address is written back as a reversal
entry against the original debit and the period’s count is decremented. You are not
charged for a recipient the provider decided not to attempt.
Retries do not double-count. A send replayed with the same Idempotency-Key and the
same canonical payload returns the original response, including the original
message_id, and produces no second debit, because there is no second message.
What is free
Test mail is never counted. Messages injected into a test inbox and messages sent from one produce real events, real error codes and real approval records, and no ledger entries at all. That is deliberate: a CI suite that exercises the whole loop on every commit should not be a line item.
The free allowance is one complimentary live inbox and 100 email units a month per verified owner identity, claimed by that identity’s first eligible workspace. Creating another workspace does not renew it, which is the detail most people test first.
The arithmetic people get wrong
Receiving costs. A support inbox that takes 40 messages and answers each of them once has spent 80 units, not 40. A thread that goes four rounds with one counterparty is eight. Any estimate built from “how many emails will the agent send” is roughly half the real figure, and closer to a third if the agent copies a human on its replies.
Auto-replies count too. An agent that acknowledges every inbound message before working on it has doubled its outbound usage for no recipient benefit, which is one of the few places where a small prompt change moves the bill.
What happens at the limit
Sending stops. Mail already received stays readable and stays exportable as JSON, and nothing is deleted for exceeding an allowance. There is no overage charge, because there is nothing to charge to: Builder at $19 a month and Team at $79 a month are proposed plans at proposed launch prices, and checkout is disabled, so no paid tier can be bought yet. The current state of that is on the pricing page, and what an inbox does with the mail it already holds is on the inbox overview.