DComTalk.com

Another part of the general spam problem is that spam blockers do not
keep spam off the network and do not cause the spammers any pain.
Most of them block spam at your machine, or at one point upstream from
your machine. So the spammers just try harder to get past them by
sending more messages, and that further congests the network.

One scheme that seemed to me to have some promise was to detect spam
in the SMTP receiving program and deliberately delay its responses
to the sending program. So that the transaction of sending a message
is stretched out far longer than normal.

jhhaynes at earthlink dot net