Nexor Sentinel is a high assurance email guard built upon an EAL4+ evaluated product and listed on NATO's NIAPC catalogue. The product validates inbound and outbound electronic messages to ensure they conform to the security policy of the protected domain. The solution provides flow control to ensure data travels in the intended direction and helps prevent covert channels. It validates data being transferred against specified security policies to reduce malware risks and prevent sensitive data leakage. Nexor Sentinel supports both SMTP and X.400 email protocols for message guarding and validation. The product includes policy-enforcing filters for message transport, content, and security. It supports signed and encrypted content types, with multiple security label formats and locations. The email guard performs message envelope checks, access control lists, and content return reporting. Content filtering capabilities include allowed attachment types and dirty word searching. Message security features cover message signatures, message encryption, and signed receipts. All messages are scanned and validated against defined security policies. Non-conformant messages are rejected and quarantined to prevent damage from outbound sensitive information loss or inbound malicious content. The system includes built-in logging and traceability for communication exchanges to ensure system integrity.