DomainKeys Identified Mail is a digital email signing/verification technology, which is already supported by some common mail providers. (For example yahoo, google, etc).
Basically DKIM means digitally singing all messages on the server to verify the message actually was sent from the domain in question and is not spam or pishing (and has not been modified).
- The sender's mail server signs outgoing email with the private key.
- When the message arrives, the receiver (or his server) requests the public key from the domain's DNS and verifies the signature.
This ensures the message was sent from a server who's private key matches the domain's public key.
For more info see RFC 6376
You may add user for opendkim or use existing one (for example: postfix)
- Generate key:
opendkim-genkey -r -s server1 -d example.com
- Create /etc/opendkim/opendkim.conf (see example in the same directory)
Domain example.com KeyFile /path/to/keys/server1.private Selector server1 Socket inet:8891@localhost UserID opendkim
- Add a DNS TXT record with your selector and public key. The correct record is generated with the private key and can be found in "server1.txt" in the same location as the private key.
- Run it with /etc/rc.d/opendkim start or add it to DAEMONS in /etc/rc.conf
into main.cf or smtpd options in master.cf
smtp inet n - n - - smtpd -o smtpd_client_connection_count_limit=10 -o smtpd_milters=inet:127.0.0.1:8891 submission inet n - n - - smtpd -o smtpd_enforce_tls=no -o smtpd_sasl_auth_enable=yes -o smtpd_client_restrictions=permit_sasl_authenticated,reject -o smtpd_sasl_path=smtpd -o cyrus_sasl_config_path=/etc/sasl2 -o smtpd_milters=inet:127.0.0.1:8891
While you're about to fight spam and increase people's trust in your server, you might want to take a look at Sender Policy Framework, which basically means adding a DNS Record stating which servers are authorized to send email for your domain.