The implementation of Secure Sockets Layer (SSL) or Transport Layer Security (TLS) for email transmission ensures that all data exchanged between the mail server and clients is encrypted. This prevents an attacker from easily viewing or modifying the information if they manage to intercept the communication, which effectively mitigates the risk of on-path attacks. The use of firewalls, while important for overall network security, does not specifically protect against the interception of data. Regularly changing passwords helps enhance security but does not directly prevent data interception. Using VPN can secure the connection from a client to a VPN endpoint, but without encrypting the emails themselves, the messages could still be vulnerable to interception once they leave the VPN endpoint.