The Queue
Not all mail messages can be delivered immediately. When delivery is delayed, sendmail must be able to save messages for later transmission. The sendmail queue comprises one or more directories that hold mail until it can be delivered. A mail message can be queued:
When the destination machine is unreachable or down. The mail message will be delivered when the destination machine returns to service.
When a mail message has many recipients. Some mail messages might be successfully delivered but others might not. Those that have transient failures are queued for later delivery.
When a mail message is expensive. Expensive mail (such as mail sent over a long-distance phone line) can be queued for delivery when rates are lower.
When (beginning with V8.11) authentication or stream encryption suffers a temporary failure to start. In this case, the message is queued for a later try.
Because safety is always primary concern. The sendmail program is configured to queue all mail messages by default, thus minimizing the risk of loss should the machine crash.
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