You are sending emails to an external domain and you get a Delivery Failure Message which says: 554. 5.7.1.
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.
IP has been included in a Real-Time Blacklist due to there is Spam coming from your Domain.
You Domain/IP has been blacklisted by the recipient.
Regarding SMG software: Make sure your SMGs Appliances are scanning Outbound Spam and Spam Filters are updated.
Follow the standard recommendations for SMTP validations on your DNS records: implement Reverse DNS and Sender Policy Framework.
Make sure that your internal Network is not compromised by making sure that Local Antivirus is installed and updated.
Make sure that your Firewall only allows connections on port 25 for your Antispam or Mail Servers.