A spool stands for Simultaneous Peripheral Operations Online. It is a list of emails, in order of when they are created, they are available for the server to send or deliver locally.
All messages go through the spool and some pass through the spool so quickly that they will not display in the spool. Most of the messages in the spool are displayed because they are large, have many recipients or are having trouble being sent to their final destination.
Below are the steps to check the spool from the SmarterMail
Login in to SmarterMail dashboard with the domain administrator user.
Click on the Spool option right side of the screen.
Once you click on the Spool option, you will get the following windows. In that window, you can see the emails which are in the spool