When you manage Postfix and have a trouble with your mail infrastructure, you may want to set in maintenance your Postfix without stopping the service. Here is a way to hold the queue, giving the time to analyze the problem and then release the queue.
Usage
You need to configure your Postfix as follow (/etc/postfix/main.cf):
Then restart Postfix. Once you're ready and want to release the queue, remove those two previous lines, restart Postfix and force the queue to release: