mailbox is full: retry timeout exceeded

If you get a bounceback with content similar to this: -

-----Original Message-----
From: Mail Delivery System
Sent: 19 May 2012 12:07
To: YOUR EMAIL ADDRESS HERE
Subject: Mail delivery failed: returning message to sender

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:

THE EMAIL ADDRESS YOU ARE TRYING TO SEND AN EMAIL TO HERE
mailbox is full: retry timeout exceeded

then the person who you are trying to send an email to has a full email account. As a result their email company is refusing to accept any emails to their email address. The only thing you can do is contact them to inform them of the problem. There is no fault with your email account, or our service, and other than contacting them to inform them of the problem there is nothing you can do.