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:

stephen@useraccount.com
   mailbox is full: retry timeout exceeded
Reporting-MTA: dns; serv01.c15792.sgvps.net

Action: failed
Final-Recipient: rfc822; stephen@useraccount.com
Status: 5.0.0

From: WMSTC <webmaster@site.com>
Subject: Invitation to be an instructor at WMSTC
Date: 5 August 2016 at 11:33:04 BST
To:  stephen@useraccount.com

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

Many thanks

Steve