Isolated inboxes are the whole point, so no paid plan meters them. You pay for retention, storage and governance — compare every detail further down.
Enterprise and Custom add SCIM provisioning, regional data residency, a signed DPA, security review, invoice or PO billing and a named customer success manager. Retention, storage and domain limits are set per contract.
Optional extras on top of any paid plan. Billed with your subscription. Inboxes are never an add-on — they are unlimited on every paid plan.