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

The `lpq` command displays the status of print queues, not email queues.

The `mailq` command, often a symbolic link to `sendmail -bp` or a similar command provided by other Mail Transfer Agents (MTAs) like Postfix, specifically prints a listing of all messages currently in the mail queue that are awaiting delivery.

`mlq` is not a standard Linux command for listing mail queues.

While `sendmail` is an MTA, the `-l` option is not the correct flag to list the mail queue; `sendmail -bp` or simply `mailq` is used.