What is the "POP before SMTP" authentication method and how is it used to defend against improper use of an SMTP server?

What will be an ideal response?

The "POP before SMTP" authentication method is another way to defend against improper use of an SMTP server. With this method, a user must be authenticated against an associated POP server before being allowed to send mail through the SMTP server. Once authenticated, the SMTP server records the IP address of the sender and allows mail to be sent from there for a period of time. Care should be taken with how long that period lasts, though; an opening could be created from which attackers at that same IP address could later send e-mail.