DKIM with postfix

Install DKIM

# yum install opendkim

Backup the original conf file.

# cp /etc/opendkim.conf{,.bak}

Open /etc/opendkim.conf and make it look like below:

AutoRestart             Yes
AutoRestartRate         10/1h
LogWhy                  Yes
Syslog                  Yes
SyslogSuccess           Yes
Mode                    sv
Canonicalization        relaxed/simple
ExternalIgnoreList      refile:/etc/opendkim/TrustedHosts
InternalHosts           refile:/etc/opendkim/TrustedHosts
KeyTable                refile:/etc/opendkim/KeyTable
SigningTable            refile:/etc/opendkim/SigningTable
SignatureAlgorithm      rsa-sha256
Socket                  inet:8891@localhost
PidFile                 /var/run/opendkim/opendkim.pid
UMask                   022
UserID                  opendkim:opendkim
TemporaryDirectory      /var/tmp

Generate keys

# mkdir /etc/opendkim/keys/example.com
# opendkim-genkey -D /etc/opendkim/keys/example.com/ -d example.com -s default
# chown -R opendkim: /etc/opendkim/keys/example.com
# mv /etc/opendkim/keys/example.com/default.private /etc/opendkim/keys/example.com/default

Open the file /etc/opendkim/KeyTable and add the following lines but with your domain instead of example.com:

default._domainkey.example.com example.com:default:/etc/opendkim/keys/example.com/default

Open the file /etc/opendkim/SigningTable and add the following line:

*@example.com default._domainkey.example.com

Open the file /etc/opendkim/TrustedHosts and make sure you have:

127.0.0.1
mydomain.com
host.mydomain.com

Note: host.mydomain.com should be your actual hostname.