You cannot send an email to some HES recipients. You receive the following non-delivery receipt (NDR) message:
554 Service unavailable; Client host [xx.xx.xx.xx] blocked using ips.backscatterer.org; Sorry xx.xx.xx.xx is blacklisted at http://www.backscatterer.org/?ip=xx.xx.xx.xx (in reply to RCPT TO command))
The NDR message shows that the recipient is using Backscatterer, which is blocking the HES outbound IP address.
Backscatterer is not the actual cause of the issue. The issue happens because the service is not being used correctly.
Backscatterer is not a blacklist or a spammerlist. It is a sender callout abuser (Ex: misdirected auto responders) list. Backscatterer recommends that you use the ips.backscatterer.org list in Safe Mode.
Emails should not be blocked based solely on this list instead the list should be used by mail server administrators to determine or tag possible spam. When solely relied upon, Backscatterer will block legitimate emails from different sources.
If this is happening to you, contact the intended email recipient and advise them to use Backscatterer in Safe Mode.