9.10. Delivery Options and Queue Management

The configuration file option " d" selects a delivery strategy to be used after a message is received by Sendmail. Note that the delivery decision is made only after a message has been completely received there is no "cut-through retransmission" involved. When Sendmail has completely received a message, that message is stored in several files in the queue, and until Sendmail releases them these files are locked against possible processing by a queue run ( la " sendmail -q").

There are three delivery options available to Sendmail once it has received an entire message. It can simply exit (" Odq"), in which case the queue files will be automatically unlocked and the message will be eligible for processing during the next queue run. Sendmail can also start an immediate delivery attempt (" Odi"), in which case the process that received the message makes a routing decision and enters the delivery phase. Finally, Sendmail can create a subprocess (via fork) to attempt a delivery in the background (" Odb"). Background and queued deliveries are useful because the original Sendmail process exits immediately after receiving the message, allowing the process that originally called Sendmail also to exit or otherwise proceed with its business.

On systems that process a large amount of mail, it is important to get the queueing strategy right or your performance will be abysmal. Depending on your...