A mail queue is a holding area where a mail server stores messages that are waiting to be delivered, either to another server or to a recipient’s mailbox. Messages may remain queued while the destination is unavailable, the server retries delivery, or a temporary configuration or service problem prevents normal processing.
Queues are important for both reliability and security monitoring. During an outage, a growing queue can signal that mail flow is disrupted; in an attack, unusual queue growth may also point to spam abuse or a compromised mail account. Administrators monitor queue size and message status to identify delays, investigate failures, and restore delivery. A queue backlog does not by itself prove an intrusion or data loss, but it can indicate that messages are not moving as expected.