Validation is the action of confirming that the email message that has been received is from a legitimate email address and not from a spammer. This works on the assumption that virtually all spam originates from automated or bulk mailer systems. This means that very rarely is any email response to the address the spam originated from looked at by a human. Automated mailing systems are very unlikely to be able to intelligently respond to requests for validation making this confirmation filter suitable for weeding out all of those unwanted spam emails.
If a bona fide email arrives from an automated mailer systems then all that needs to be done is the addition of an entry to the relevant address book containing the email address the mail comes from and the filter will not be tripped by mail from that address. Additionally, the message can be moved from Quarantine to the inbox with the push of a button.
To allow this to work all messages that come from an unknown source are automatically transferred to a special system-generated folder called the Quarantine folder. The sender of the message is sent an automated response containing a token requesting validation. If a response to this Confirmation email is received, this validates the email address and the message is removed from the Quarantine folder to be transferred into the Inbox. If the sender sends multiple messages to the users inbox, they will only get one confirmation message. Once they respond to that confirmation all their messages will be moved to the inbox. Note that the sender must reply to the confirmation from the same email address that they sent the original message from for the address to be validated.