Server Fault is a question and answer site for system and network administrators. It only takes a minute to sign up. Connect and share knowledge within a single location that is structured and easy to search. As suggested in How do you detect a spambot on your network? Something related is being talked here but not sure of the Iptables rules. Run Postfix under its own user account. It should already be doing so, on any sane system.
What are SPF and DKIM Records?
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Create an SPF Record in DNS
We can send and receive email using a desktop email client. I have seen it used by facebook. Some folks might think that -all will be better as it will reject emails from untrusted hosts. To check if your SPF record is propagated to the public Internet, you can use the dig utility on your Linux box like below. On Ubuntu, you need to install the bind9-dnsutils package in order to use dig command: sudo apt install bind9-dnsutils. Append the following lines at the end of the file. The first line specifies the Postfix policy agent timeout setting. The following lines will impose a restriction on incoming emails by rejecting unauthorized email and checking SPF record. Next time, when you receive an email from a domain that has an SPF record, you can see the SPF check results in the raw email header.
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