What is the purpose of the command mailq?

A.
It fetches new emails from a remote server using POP3 or IMAP.

B.
It is a multi-user mailing list manager.

C.
It is a proprietary tool contained only in the qmail MTA.

D.
It queries the mail queue of the local MTA.

E.
It is a command-line based tool for reading and writing emails.