The qmail Handbook

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.