Messages are not delivered to an external domain and Message Audit Logs show a final delivery status with the "554 5.7.1" status code.
SMTP Code 554. 5.7.1 is a rejection from the Recipient Domain. SMTP Code 554. 5.7.1 means 'Not allowed'. The address in 'Mail From' appears to have insufficient submission rights, or is invalid, or is not authorized with the authentication used; the address in an RCPT TO command is inconsistent with the permissions given to the user; the message data is rejected based on the submitting user. A 5xx level status code indicates a "terminal" status and SMG will not attempt to redeliver the message.
IP has been included in a Real-Time Blacklist due to Spam coming from your domain.
Your domain/IP has been blacklisted by the recipient.
Please review TECH122730 for best practices on configuring Messaging Gateway for both inbound and outbound traffic.
Ensure that Messaging Gateway is scanning outbound messages for Spam and Spam Filters are updated.
Follow the standard recommendations for SMTP validations on your DNS records: implement Reverse DNS and Sender Policy Framework.
Ensure that your internal Network is not compromised by making sure that Local Antivirus is installed and updated.
Ensure that the Firewall only allows connections on port 25 for your Antispam or Mail Servers and that outbound connections to port 25 is limited to SMG or other, trusted, mail sources.