Hi
Here some nice howto to setup opendkim with postfix
Install and configure opendkim:
aptitude install opendkim
mkdir -p /etc/opendkim/keys/yourdomain.tld
cd /etc/opendkim/keys/yourdomain.tld
opendkim-genkey -r -d yourdomin.tld
vim /etc/opendkim.conf
AutoRestart Yes
AutoRestartRate 10/1h
Syslog yes
LogWhy yes
SyslogSuccess yes
UMask 002
Socket inet:8891@localhost
KeyTable refile:/etc/opendkim/keytable
SigningTable refile:/etc/opendkim/signingtable
vim /etc/opendkim/keytable
default._domainkey.yourdomain.tld yourdomain.tld:default:/etc/opendkim/keys/yourdomain.tld/default.private
vim /etc/opendkim/signingtable
*@yourdomain.tld default._domainkey.yourdomain.tld
vim /etc/postfix/master.cf
smtp inet n - - - - smtpd
-o smtpd_milters=inet:127.0.0.1:8891
smtps inet n - - - - smtpd
-o syslog_name=postfix/smtps
-o smtpd_tls_wrappermode=yes
-o smtpd_sasl_auth_enable=yes
-o smtpd_milters=inet:127.0.0.1:8891
-o smtpd_client_restrictions=permit_sasl_authenticated,reject
/etc/init.d/postfix restart
/etc/init.d/opendkim restart
cat /etc/opendkim/keys/yourdomain.tld/default.txt
add this record to your dns zone of yourdomain.tld
Hint: In ubuntu 12.04 opendkim-genkey has a bug which generates an invalid dkim public key in the default.txt. Here the example:
default._domainkey IN TXT "v=DKIM1;=rsa; p=MIGfMA0GC .... Q7GWwsbQIDAQAB" WRONG
default._domainkey IN TXT "v=DKIM1; k=rsa; p=MIGfMA0GC .... Q7GWwsbQIDAQAB" RIGHT
You can also install a ubuntu backport which does not have the problem
https://bugs.launchpad.net/ubuntu/+source/opendkim/+bug/1083503
Hint:
To verify your dkim install you can use any gmail account.
Here Gmails help for this http://support.google.com/mail/bin/answer.py?hl=en&answer=180707
“mailed-by yourdomain.tld” -> Means your SPF Record is valid
“signed-by yourdomain.tld” -> Means your DKIM Setup is valid
Have fun!