why- Open BSD .rocks

spamd(8) (∞)

The spamd spam deferral daemon rejects fake emails. It is designed for efficiency, so that the receiving host is not slowed down.

spamd divides sending hosts into three types:

Blacklisted hosts are diverted to spamd and tarpitted. In other words, 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 smtpd(8).

Greylisted hosts are diverted to spamd if it is not yet known whether they are likely spammers. They are sent a temporary failure message when they try to deliver mail.