What is a potential drawback of callback verification in email filtering?

Callback verification is a method used in email filtering where the system attempts to verify the sender's email address by contacting the original mail server for that address. The correct option highlights a key limitation of this approach: it can struggle with forged addresses that are valid.

In many cases, spammers use addresses that appear legitimate or disguise their true sender information through techniques like spoofing. When the email filtering system reaches out to verify these forged addresses, it may receive responses indicating that the address is valid. This leads to the unintended consequence of allowing spam through, as the filtering system cannot differentiate between a legitimate sender and a forged one that has been successfully masquerading as a valid user.

This challenge underscores the technology's reliance on the integrity of sender addresses, which can be exploited by those maliciously crafting emails, thereby diminishing the overall effectiveness of the callback verification method in filtering out unwanted spam.