outboxes

An outbox is a location used to hold outgoing messages or data items before they are transmitted or processed. The term is used in multiple domains, including email clients, messaging systems, and software architectures. In each context, the outbox serves as a buffer or queue that coordinates the delivery or processing of outgoing work.

In email clients and mobile apps, the Outbox folder temporarily stores messages that are waiting to be sent, such as when a device is offline or when transmission is scheduled. Once a message is successfully transmitted, it typically moves to the Sent folder. Messages may remain in or retry from the Outbox if sending fails, and some systems provide a “send later” option that uses the outbox to schedule delivery.

In software architecture, the Outbox pattern is a design for reliable event publication in distributed systems. A transactional write to the primary database also writes a record to an outbox table, indicating which events should be published. A separate process reads unprocessed outbox entries and publishes the corresponding messages to a message broker, then marks them as processed. This approach helps achieve durable, at-least-once or exactly-once delivery, depending on implementation, while preserving transactional boundaries.

Outboxes may be implemented as filesystem folders, database tables, or specialized messaging queues, depending on the system. They are distinct from Inbox and Sent concepts and require explicit handling for cleanup, error recovery, and idempotence to avoid duplicates or data leakage.