The _______ command prints a list of email that is currently in the queue waiting for delivery.

Answer: mailq

Explanation:
From the man pages:
mailq List the mail queue. Each entry shows the queue file ID, message size, arrival time, sender, and the recipients that still need to be delivered. If mail could not be delivered upon the last attempt, the reason for failure is shown. The queue ID string is followed by an optional status character:
* The message is in the active queue, i.e. the message is selected for delivery.
! The message is in the hold queue, i.e. no further delivery attempt will be made until the mail is taken off hold.
This mode of operation is implemented by executing the postqueue(1) command.

This is identical to sendmail -bp.