Content filter is a component on a Cisco Secure Email Gateway that must be configured to catch attachments, including credit card numbers, and hold them for review until further action is taken. Content filter allows you to define rules based on message content and apply actions such as quarantine, encrypt, or modify. Reference = [User Guide for AsyncOS 12.0 for Cisco Email Security Appliances - GD (General Deployment) - Content Filters [Cisco Secure Email Gateway] - Cisco]

The reason why the CEO did not receive an important message expected from a trusted sender after adding them to a safelist is because end-user safelists apply to antispam engines only. End-user safelists are lists of sender addresses or domains that end users can create and manage through their quarantine accounts or email clients. End-user safelists allow end users to accept or exempt messages from certain senders or domains from being identified as spam by the antispam engines. However, end-user safelists do not affect other filtering engines such as antivirus, outbreak filters, or content filters. Reference = User Guide for AsyncOS 12.0 for Cisco Email Security Appliances - GD (General Deployment) - Safelists and Blocklists [Cisco Secure Email Gateway] - Cisco

If you are unable to access the user interface of the Cisco Secure Email Gateway appliance after manipulating the subnet mask, you can use the serial or console port to perform the initial configuration. The serial or console port provides a command-line interface that allows you to configure basic network settings such as IP address, subnet mask, gateway, and hostname3. Reference = User Guide for AsyncOS 12.0 for Cisco Email Security Appliances - GD (General Deployment) - Configuring Network Settings [Cisco Secure Email Gateway] - Cisco

Explanation
If you configure multi-layer anti-virus scanning, the Cisco appliance performs virus scanning with the McAfee engine first and the Sophos engine second. It scans messages using both engines, unless the McAfee engine detects a virus. If the McAfee engine detects a virus, the Cisco appliance performs the anti-virus actions (repairing, quarantining, etc.) defined for the mail policy.