If emails with the code 554 5.7.1 are being rejected when sending via Hornetsecurity, the sending IP address is not stored in the control panel. To determine, which IP address your system is sending from, have the source IP displayed in the Email Live Tracking.
As soon as you have found the source IP, you can enter it in the corresponding field as described here.
Please note:
All newly added IP addresses are distributed to the Hornetsecurity servers within a period of 30 minutes. During this period, emails can still be rejected. If emails with the error code above continue to be rejected after 30 minutes, please contact your Hornetsecurity partner.