Spamd is a fake sendmail(8)-like daemon which rejects false mail. It is designed to be very efficient so that it does not slow down the receiving machine.
spamd considers sending hosts to be of three types:
blacklisted hosts are redirected to spamd and tarpitted i.e. they are communicated with very slowly to consume the sender's resources. Mail is rejected with either a 450 or 550 error message. A blacklisted host will not be allowed to talk to a real mail server.
whitelisted hosts do not talk to spamd. Their connections are instead sent to a real mail server, such as sendmail(8).
greylisted hosts are redirected to spamd, but spamd has not yet decided if they are likely spammers. They are given a temporary failure message by spamd when they try to deliver mail.