Which of the following commands should be used to print a listing of emails in the system's mail queue?

`sendmail -l` is not a standard command or option to list the mail queue; `sendmail` itself is a Mail Transfer Agent.

`lpq` is used to examine the spool queue for printing jobs, not email messages.

The `mailq` command is specifically designed to show the current contents of the system's mail queue, providing information about messages that are waiting to be sent, their sender, recipient, size, and status. It is a utility commonly associated with MTAs like Postfix or Sendmail.

`mlq` is not a standard command for displaying the mail queue.