What technique is Leonard trying to employ here to stop SPAM?

Leonard is a systems administrator who has been tasked by his supervisor toslow down or lessen the amount of SPAM their company receives on a regular basis. SPAM being sent to company email addresses has become a large problem withinthe last year for them. Leonard starts by adding SPAM prevention software at the perimeter of the network. He then builds a black list, white list, turns on MX callbacks, and uses heuristics to stop the incoming SPAM. While these techniques help some, they do not prevent much of the SPAM from coming in. Leonard decides to use a technique where his mail server responds very slowly to outside connected mail servers by using multi-line SMTP responses. By responding slowly to SMTP connections, he hopes that SPAMMERS will see this and move on to easier a nd faster targets. What technique is Leonard trying to employ here to stop SPAM?

A.
He is using the technique called teergrubing to delay SMTP responses and hopefully stop SPAM.

B.
This technique that Leonard is trying is referred to as using a Sender Policy Framework to aid in SPAM prevention.

C.
Leonard is trying to use the Transparent SMTP Proxy technique to stop incoming SPAM.

D.
To stop SPAM, Leonard is using the technique called Bayesian Content Filtering.