InterScan Messaging Security Virtual Appliance (IMSVA) shows "Not UTF-8 encoded string" in the Subject field of the Message Tracking Logs. It occurs when there are non-UTF-8 characters in the subject of an email.
This is an expected behavior. If a non-ASCII character is included, the sender needs to specify which encoding format is being used.
In this case, as the encoding format is not specified, InterScan Messaging Security Virtual Appliance does not know the original encoding. Therefore, it cannot display the character properly.