You have received a "content too deep" error on a message that contains XML file(s).
At first glance the message does not appear to have multiple containers or any levels.
Error message is thrown on a message with an XML attachment stating "content too deep."
When an XML file is attached it could generate multiple MIME parts in the message. Each MIME part is considered to be a separate container so if this number is exceeding the maximum container scan depth limits in the Symantec product in question you might see this error.
It is not advisable to exclude XML files from scanning.
You can workaround the issue by either compressing the attachments into a single file with a password.