Callback verification is aimed at:

Callback verification is a process used primarily to confirm the legitimacy of a sender's email address, especially in cases where there is suspicion of forgery or spoofing. When a system employs callback verification, it essentially sends a request to the sender's email server to verify that the specified email address is valid and that the email is not coming from a fraudulent source.

This method helps prevent malicious activities, such as phishing scams, by ensuring that the originating address is indeed associated with a legitimate entity. By confirming the legitimacy of the sender's identity, callback verification plays a crucial role in maintaining the integrity of email communications and protecting recipients from deceptive practices.

The other options do not accurately describe the purpose of callback verification. Filtering emails before they arrive involves a different approach focused on sorting and managing incoming messages. Optimizing server response time relates to improving the efficiency of data requests and services. Similarly, blocking all emails without verification applies a more generalized restriction rather than the targeted legitimacy check that callback verification performs.