Postfix/Greylisting

Greylisting is a process in which systems attempting to connect to your server to deliver mail for the first time are treated differently to known peers, delaying their mail. If the sending mail server is standards-compliant, it will re-send the e-mail, and the server will accept it. Most spam mailers, however, don't re-send the mail, and so the spam is blocked. Servers that re-send the mail will be added to a white list, and will not be delayed in future. This means that the first e-mail from a given sender will be delayed, but subsequent ones will not be.

Installation
Greylisting for postfix is typically implemented by using the the package, so first install that:

Setup
Next, we need to start it, and set it to start automatically.

Now we have to tell postfix to use it, by adding the  entry to the existing   directive in your   file, as follows.

Finally, tell postfix to reload its configuration for the changes to take effect.