Egress filtering is designed to:

Egress filtering is a network security measure focused on monitoring and controlling the traffic that is allowed to leave a network. This is essential for preventing sensitive data from being transmitted outside the organization without authorization and for identifying potential security threats within outbound traffic.

The correct choice emphasizes the role of egress filtering in performing spam filtering on messages that are sent from within the network to external destinations. By examining out-going messages, egress filtering can protect against the potential for internal systems being compromised to send spam or other malicious content to external recipients. This is important for maintaining the integrity of the organization's communication and preventing the spread of spam or malware.

In the context of the other options, filtering inbound email messages pertains to ingress filtering, which focuses on incoming traffic and threats. Blocking all outgoing communication is an extreme measure that would hinder normal business operations and is not the objective of egress filtering. Lastly, identifying spam based on incoming reputation relates to assessing the trustworthiness of messages arriving at the network rather than those being sent out. Thus, the primary function of egress filtering aligns closely with the highlighted correct choice.