Mail Delivery System

Hi I am working through CoursePress and when sending invites to new Instructors I get the following;

This message was created automatically by mail delivery software.

A message that you sent could not be delivered to one or more of its
recipients. This is a permanent error. The following address(es) failed:
mailbox is full: retry timeout exceeded
Reporting-MTA: dns;

Action: failed
Final-Recipient: rfc822;
Status: 5.0.0

From: WMSTC <>
Subject: Invitation to be an instructor at WMSTC
Date: 5 August 2016 at 11:33:04 BST

Note, I have changed the email accounts as so not to show real data.

Many thanks