What are email queues?

An email queue is a holding area where messages wait before delivery or during retry attempts. When your MTA can't deliver immediately (recipient server busy, temporary error, rate limiting), the message stays in queue until the next attempt.

Queues serve several purposes:

Buffer for volume spikes: When you send faster than recipients can accept, messages queue rather than fail.

Retry management: Messages that receive temporary errors (4xx codes) queue for later retry rather than bouncing immediately.

Delivery pacing: Queues enable throttling by controlling how fast messages leave for each destination.

A typical MTA maintains different queue types: active queue (being processed now), deferred queue (waiting for retry), and hold queue (manually paused). Messages move between queues based on delivery results.

Queue health matters. Growing queues indicate delivery problems. Stale messages suggest configuration issues. Monitoring queue depth and age helps catch problems before they escalate.

The queue is your ship's hold. Cargo waits there until the destination port is ready to receive it.