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DKIM is an authentication framework which stores public-keys in DNS and digitally signs emails on a domain basis.

rpm --nodeps http://www.c-corp.net/linux/centos/5/general/RPMS/i386/dkim-milter-2.2.1-1.i386.rpm
mkdir /etc/dkim-milter
chown dkim-milt.dkim-milt /etc/dkim-milter
chmod 700 /etc/dkim-milter
chgrp postfix /var/run/dkim-milter
chmod 770 /var/run/dkim-milter

Generate The Keys

Enter the following command to generate your private key:

openssl genrsa -out rsa.private 768

Enter the following command to generate your public key:

openssl rsa -in rsa.private -out rsa.public -pubout -outform PEM

mv rsa.private /etc/dkim-milter/_default.key.pem

Edit the file /etc/sysconfig/dkim-milter

USER="dkim-milt"
PORT=local:/var/run/dkim-milter/dkim.sock
SIGNING_DOMAIN=""
SELECTOR_NAME="default"
KEYFILE="/etc/dkim-milter/${SIGNING_DOMAIN}_${SELECTOR_NAME}.key.pem"
SIGNER=yes
VERIFIER=yes
CANON=simple
SIGALG=rsa-sha1
REJECTION="bad=r,dns=t,int=t,no=a,miss=r"
EXTRA_ARGS="-h -l -D"


Restart milter service

service dkim-milter start

Edit /etc/postfix/main.cf:
smtpd_milters = unix:/var/run/dkim-milter/dkim.sock
non_smtpd_milters = unix:/var/run/dkim-milter/dkim.sock