Address Sender Verification
One of the best methods of restricting SPAM is to require address verification. This means that Postfix will initiate a SMTP session with the client’s server to verify that it is a legitimate address. This takes time and resources but…it a very effective way to deal with SPAM. You will need to add the reject_unverified_sender option.
There is a way to enhance this process. One thing that Postfix will do is to cache the addresses it checks out and saves them in memory. This is great because the system will not have to look the same address up again…unless you restart the server as the memory will lose the addresses. However, you can tell Postfix to write the addresses to a map file that will allow Postfix to cache them permanently. Use the address_verify_map feature to make this work.
address_verify_map = btree:/var/spool/postfix/verified_senders
If you did not want to cache the negative sender addresses you can use this parameter.
address_verify_negative_cache = no
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