Which of the following is used to prevent unwanted emails from reaching an inbox?

A spam gateway is an email-security system positioned in the mail-delivery path. It examines incoming messages and attempts to identify spam, phishing messages, malicious attachments, suspicious links, spoofed senders, and other unwanted content.

Depending on its configuration, a spam gateway can:

Reject the message.

Quarantine it for review.

Mark it as spam.

Deliver it to a junk-mail folder.

Remove or block malicious content.

Because the question specifically asks about preventing unwanted email from reaching an inbox, a spam gateway is the most precise answer.

A proxy server commonly intermediates web or application requests.

An intrusion detection system monitors activity and generates alerts but does not primarily filter email.

A firewall filters network connections and ports but normally does not perform detailed email-content classification.

CompTIA objective 2.3 explicitly identifies spam gateways as network-based internet appliances. Microsoft’s anti-spam documentation explains that spam filtering identifies and separates unwanted email from legitimate messages