====== Using Greylisting in Postfix  ======

Greylisting can be a pretty effective method for reducing the amount of spam you receive.  It works by temporarily rejecting emails.  Valid email servers will then retry delivery, and be allowed through while spammers will (hopefully) give up.

  - Start with a fully updated and configured Postfix server.
  - Install Postgrey:<code bash>
sudo apt -y install postgrey
</code>
  - Configure Postfix to use Postgrey:<code bash>
cp /etc/default/postgrey /tmp
sed -i 's/POSTGREY_OPTS="--inet=10023"/POSTGREY_OPTS="--inet=127.0.0.1:10023 --delay=180"/' /tmp/postgrey
sudo mv /tmp/postgrey /etc/default/

SRR=$(postconf -p smtpd_recipient_restrictions | sed 's/smtpd_recipient_restrictions = //g')

if [ -z "$SRR" ]; then
     SRR="check_policy_service inet:127.0.0.1:10023";
else
     SRR="$SRR,check_policy_service inet:127.0.0.1:10023"
fi

sudo postconf -e "smtpd_recipient_restrictions = $SRR"
</code>
  - Restart Postfix and Postgrey:<code bash>
sudo systemctl restart postfix
sudo systemctl restart postgrey
</code>