"Undelivered Mail Returned To Sender"

"Undelivered Mail Returned To Sender"

I keep getting the error message below when trying to email one specific email address and just wanted to make sure that there is no problem at my end, and that they have a problem.

Error message:

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. 

xxxxxx@xxxxxx-xxx.co.uk Invalid Address, ERROR_CODE :550, ERROR_CODE :Plusnet: REJECTED: Relay not permitted 


Received:from mail.zoho.com by mx.zohomail.com 
    with SMTP id 1376647777368450.4934387356742; Fri, 16 Aug 2013 03:09:37 -0700 (PDT) 
Message-ID:<140869a0c17.-3155054820027647835.8125619290688214448@xxxxxxxxxxx.com> 
Date:Fri, 16 Aug 2013 11:09:37 +0100 
From:Chuck Rockwalker <xxxxx@xxxxxxxxxxx.com> 
User-Agent:Zoho Mail 
To:Trevor Wayman <xxxxxx@xxxxxx-xxx.co.uk> 
Subject:RE: xxxxxx xxxxxx 
Content-Type:multipart/alternative; 
    boundary="----=_Part_257990_361012502.1376647777301"