There was a SMTP communication problem with the recipient's email server. Please contact your system administrator.
<XXXXXX.co.nz (my website) #5.5.0 smtp;550-X-Warning: XXX.XXX.XXX.X (IP Address) is listed at zen.spamhaus.org (127.0.0.11:>
I checked this website and it seems to be some sort of spam blocking site which blacklists IP addresses. I have sent no spam from my exchange server (and have checked through the system for other evidence of spam being sent - which came back clean) so I am wondering why I have been blocked, and how to unblock it.