An information security procedure that indicates a requirement to sandbox emails means that the emails need to be isolated and tested for malicious content. This is because sandboxing is a technique that creates a virtual or isolated environment, where suspicious or untrusted emails can be executed or analyzed without affecting the rest of the system or network. Sandboxing helps to detect and prevent malware, phishing, or spam attacks that may be embedded in emails, and protect the users and the organization from potential harm. The other options are not what sandboxing emails means, but rather different concepts or techniques that are related to information security, such as encryption and nonrepudiation (A), backup and recovery (B), or firewall and delivery (D).