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qmail includes the following queue management features:
Instant handling of messages added to queue. New messages are always delivered immediately, subject to resource availability.
Parallelism limits. The number of simultaneous local and remote deliveries is limited and configurable.
Split queue directory. Some Unix file systems experience significant slow downs with large directories. qmail splits the queue into a configurable (at compilation) number of subdirectories to keep the number of files per directory low.
Quadratic retry schedule. Undelivered messages in the queue are retried less frequently as they age-the longer a host has been unreachable, the less likely it is to be reachable soon.
Independent message retry schedules. Each message has its own retry schedule. If a long-down host comes back, qmail won't immediately flood it with a huge backlog.
Automatic safe queuing. No mail is lost if the system crashes.
Automatic per-recipient checkpointing. Each successful delivery of a message to multiple recipients is recorded, preventing the sending of duplicates in the event of a crash.
Automatic queue cleanups. Interrupted queue injections can leave partially injected messages in the queue. qmail-send automatically cleans these out after 36 hours. qmail-clean removes messages after successful delivery.
Queue viewing. qmail-qread displays the current contents of the queue.
Detailed delivery statistics. The qmailanalog package analyzes the qmail-send logs and produces delivery statistics.
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