The Mail Queue

In a normal configuration, sendmail sits in the background waiting for new messages. When a new connection arrives, a child process is invoked to handle the connection, while the parent process goes back to listening for new connections.

When a message is received, the sendmail child process puts it into the mail queue (usually stored in /var/spool/mqueue). If it is immediately deliverable, it is delivered and removed from the queue. If it is not immediately deliverable, it will be left in the queue and the process will terminate.

Messages left in the queue will stay there until the next time the queue is processed. The parent sendmail will usually fork a child process to attempt to deliver anything left in the queue at regular intervals.

Mark D. Roth <>