Which type of email traffic must pass through Mimecast for a HEX platform customer?

The correct choice indicates that all incoming traffic must pass through Mimecast for a HEX platform customer. This requirement ensures that all incoming emails are processed through Mimecast's advanced security features. By routing all incoming traffic through Mimecast, customers benefit from enhanced security measures such as anti-spam filtering, malware detection, and email threat protection. This comprehensive handling of incoming messages helps organizations safeguard their environments against potential risks associated with phishing attacks and malicious content.

In contrast, the other options suggest limited routing of email traffic or optional participation in the service, which does not align with the security posture that Mimecast aims to provide. Ensuring that all incoming traffic is monitored and filtered allows for better oversight and management of threats, reducing the likelihood of harmful emails reaching the organization’s inboxes.