What is email greylisting by the mail transfer agent?

Greylisting works by temporarily rejecting email from any sender (specifically, the triplet of sender IP, sender email address, and recipient email address) that has not been seen before, expecting legitimate mail servers to retry later, while most spammers will not.

Returning emails that are potential phishing attempts is a function of content filtering or advanced threat protection, not greylisting.

Greylisting *temporarily denies* emails from unknown senders, it does not *allow* them temporarily; it delays them to verify legitimacy.

Quarantining emails from outside the organization is a general spam or policy enforcement action, not the specific mechanism of greylisting which relies on temporary rejection and retry.