Greylisting is a great way for fighting spams, the basic idea out of it is that spammers mail servers are not respecting RFC standards specifications which basically says that when an email could not be delivered, the mail server should try again later on. By sending so many emails, spammers can't afford to spend to much resources on resending emails when they could not be delivered, so if the email could not be delivered in the first place, they won't send it back to you.

From this ideas, greylisting simply reject any untrusted mail domain by giving a 450 response code, which means "I can't deal with your request now, please try again later". As spam mail server are not usually RFC compliant, they won't try back and therefore you won't get the spam.

At Debuntu.

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