The best recommendation to make is to setup a network firewall. A network firewall is a device or a software that monitors and controls the incoming and outgoing network traffic based on predefined rules and policies. A network firewall can protect the email, web, and FTP servers from attacks by blocking or filtering the malicious or unwanted traffic, such as denial-of-service, port scanning, or brute force attacks. A network firewall can also log and audit the network activity and provide alerts and reports on the security incidents.
The other options are not as effective as setting up a network firewall, as they either do not protect all the servers, do not control the network traffic, or do not prevent the attacks. References: CISSP - Certified Information Systems Security Professional, Domain 4. Communication and Network Security, 4.2 Secure network components, 4.2.1 Establish secure communication channels, 4.2.1.2 Transmission methods; CISSP Exam Outline, Domain 4. Communication and Network Security, 4.2 Secure network components, 4.2.1 Establish secure communication channels, 4.2.1.2 Transmission methods