Question: Why would an email message have a "[WARNING - NOT VIRUS SCANNED]" string added to the subject line?
This is usually caused as a result of the virus scanning process which is part of the Symantec Brightmail email scanner (Spam Quarantine).
A common cause for this message is an unscannable message. A message can be unscannable for viruses if it is too large or too complex. One example of a complex messages is one that has a zip file with many nested folders.
Be cautious, when opening unscanned messages or attachments.
Symantec has a knowledge base article with a little more information.