Why are my emails being rejected by Zoho's SpamBouncer?

Why are my emails being rejected by Zoho's SpamBouncer?

Body of rejection email:-

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. 

phil.mcconkey@sunpower-uk.com  Error, ERROR_CODE :554, ERROR_CODE :SpamBouncer identified SPAM, message permanently rejected (#5.3.0) 


Received:from mail.zoho.com by mx.zohomail.com 
    with SMTP id 1422446146523288.7800458778029; Wed, 28 Jan 2015 03:55:46 -0800 (PST) 
Message-ID:<14b3061c31f.c05 b3e7443322.6773639527520626740@hytronikuk.com
Date:Wed, 28 Jan 2015 11:55:46 +0000 
From:Dave Stockman < daves@hytronikuk.com
User-Agent:Zoho Mail 
To:< phil.mcconkey@sunpower-uk.com
Cc:"Richard Faulkner" < richardf@hytronikuk.com
Subject:Fwd: RE: New PO # 46825, 46826 - Confirmation? 
Content-Type:multipart/alternative; 
    boundary="----=_Part_59070_1767189682.1422446146448"