This error might be because of the IP has been included in a Real-Time Blacklist due to there is Spam coming from your Domain.
Please follow the below points to resolve the problem.
1. Regarding SMG software: Make sure your SMGs Appliances are scanning Outbound Spam and Spam Filters are updated.
2. Follow the standard recommendations for SMTP validations on your DNS records: implement Reverse DNS and Sender Policy Framework.
3. Make sure your internal Network is not compromised by making sure Local Antivirus are installed and updated.
4. Make sure your Firewall only allows connections on port 25 for your AntiSpam or Email Servers.
5. Check the standard recommendations at: http://ipremoval.sms.symantec.com/lookup/