Essentially there was an issue that caused the filtering of the message to not occur. Unfortunately, there is not enough information provided to determine what happened. The message was likely filtered on another pass.
If it is an error that is not persisting and only lasted a short amount of time, then you may be able to ignore it. The Symantec Messaging Gateway is designed to handle errors like this.
If it is an error that you are seeing often and persisting, then your best bet would likely be to generate a diagnostics package (with the "all logs" option) and contact support:
http://www.symantec.com/business/support/index?page=content&id=howto25855
If you are not on the latest version of the Messaging Gateway (9.5.3), then you may want to upgrade as a first step.