Queues are commonly used in network systems. For example, e-mail is placed in queues while it is waiting to be sent and after it arrives at the recipient’s mailbox. A problem occurs, however, if the...

Queues are commonly used in network systems. For example, e-mail is placed in queues while it is waiting to be sent and after it arrives at the recipient’s mailbox. A problem occurs, however, if the outgoing mail processor cannot send one or more of the messages in the queue. For example, a message might not be sent because the recipient’s system is not available.