Outbox

An outbox is a container for outbound messages or data in communication systems and software. In email clients and messaging apps, it refers to a folder or queue where messages waiting to be sent are stored. It contrasts with the inbox, where received messages reside, and with Sent or Delivered, where successfully dispatched messages are archived.

In email workflows, messages are composed and placed into the Outbox. If an internet connection is unavailable, the app keeps them there until a connection is established; once the SMTP server accepts the message, it is moved to the Sent folder. If delivery fails, the message can remain in the Outbox or be moved to a Drafts or Failed area, depending on the client.

In software engineering, the Outbox pattern is a reliability pattern used in distributed systems. When a service performs a business transaction, it writes a record to an outbox table within its own database and publishes events to a message broker from that table. A separate process reads the outbox, publishes the messages, and marks them as dispatched. This helps maintain data consistency across services and reduces the risk of lost events due to partial failures.

Other considerations include offline support, synchronization across devices, privacy and security of outgoing data, and retention policies for old outbound messages. The exact behavior of an Outbox can differ across platforms and applications, but the core idea is to provide a temporary holding area for outbound communications until delivery or processing is complete.