White Listing vs Grey Listing
White listing is a process of adding an email to an approved sender list, so emails from that sender are never moved to the spam folder. This tells an email server to move messages to the inbox directly.
Greylisting is a method of protecting e-mail users against spam. A mail transfer agent (MTA) using greylisting will “temporarily reject” any email from a sender it does not recognize. If the mail is legitimate, the originating server will try again after a delay, and the email will be accepted if sufficient time has elapsed.
Visit the following resources to learn more: