I seem to need to refer to this more often than I would have imagined: Postfix Mail Queue Management.
Today I learned about the ‘mailq’ and ‘postqueue’ commands.
The ‘mailq’ command will list the contents of the Postfix mail queue.
The command ‘postqueue -f’ will process all items in the Postfix mail queue.
There’s more information in this article, View the Sendmail or Postfix mail queue.