Users are receiving a non-delivery receipt (NDR) when sending an email to a Symantec Email Security.Cloud customer.
501 Connection rejected by policy [7.1], please visit www.messagelabs.com/support for more details about this error message
The sender IP address has been listed in third party email blacklist.
The sender's administrator would need to check their IP and request de-listing from any blacklists.
Once the IP has been removed from these lists, the sender domain can try to send the email once again.
The following sites could be used to check for any blacklisting: