According to the [Cisco Secure Email User Guide], Non-Viral threat detection is a feature of Outbreak Filters that detects and blocks email messages that contain non-viral threats such as phishing, fraud, or social engineering[1, p. 25]. To use this feature, you need to enable either AntiSpam or Intelligent Multi-Scan on your Cisco Secure Email Gateway, as these features provide the necessary scanning and filtering capabilities for Non-Viral threat detection[1, p. 26].

The other options are not valid because:

A. Non-Viral threat detection does not require Antivirus or AMP enablement to properly function. Antivirus and AMP are features that detect and block email messages that contain viral threats such as malware or ransomware[1, p. 27-28].

B. The Outbreak Filters option Graymail Header does not affect Non-Viral threat detection. Graymail Header is an option that allows you to add a header to email messages that are classified as graymail, which are messages that are not spam but may be unwanted by some recipients, such as newsletters or promotions[1, p. 25].

D. The Outbreak Filters option URL Rewriting does not affect Non-Viral threat detection. URL Rewriting is an option that allows you to rewrite the URLs in email messages to point to a Cisco proxy server, which can scan the URLs for malicious content and redirect the users to a warning page if needed[1, p. 25].