The send-first model
Transactional email products are excellent when an application emits receipts, alerts, password resets, or campaigns. The API call is the product: accept a message, deliver it, and report status. Inbound processing is often a domain-level webhook that leaves storage, identity, threading, and tenant routing to your application.
The inbox-first model
An inbox API makes the mailbox the primary resource. Creating an agent creates an address and a durable place for replies. Messages belong to threads, credentials can be scoped to an identity or workspace, and events refer to stable resources rather than one-off payloads.
Choose based on the conversation
Use a send-first API for one-way application mail. Use an inbox API when the recipient can answer, the agent must remember prior messages, the identity must persist, or a new mailbox is provisioned for each customer or agent. Many products use both: transactional infrastructure for application notifications and agent inboxes for autonomous conversations.
Frequently asked questions
Is SMTP an inbox API?
No. SMTP is a message-transfer protocol. A mailbox also needs receiving, storage, retrieval, identity, threading, and access control.
Can a transactional email webhook become an inbox?
Yes, but your application must add address provisioning, durable storage, MIME parsing, threading, retries, search, and tenant isolation.