The mail queue is a directory where outbound emails sit (queue) while awaiting delivery.

Generally mail piles up in the queue when they can not be delivered.

You can use the mailq command to display a summary of the mail messages queued for future delivery. Type the following command:
# mailq

/var/spool/mqueue has all the mail in the queue.

Delete the files in it or achive it. You will win free inodes deleting mqueue.

Most common, the relay server set up in is not accepting connections because it has not been configured for relay.