How message queue works to ensure delivery just at the right time

How message queue works to ensure delivery just at the right time

New messages  entering to the Postfix queue is written by the cleanup service into the "incoming" queue. New queue files are created owned by the "postfix" user with an access  mode of  0600. Once a queue file is ready for further processing the cleanup service changes the queue file mode to 0700 and notifies the queue manager of new mail arrival. The incomplete queue files ignored by the  manager whose mode is 0600, even these are still being written by cleanup. The queue manager scans the incoming in to the active queue if its resource limits have not been exceeded. By default, the active queue accommodates at most 20000 messages. Once the active queue message limit is reached, the queue manager stops scanning the incoming queue. Under normal conditions the incoming queue is nearly empty, with the queue manager able to import new messages into the active queue as soon as they become available.  The in_flow_delay parameter is used to clamp the input rate when the queue manager starts to fall behind. The cleanup service will pause for $in_flow_delay  seconds before creating a new queue file if it cannot obtain a "token" from the queue manager.