I have written a much newer, clearer and better article on DomainKeys signing email `here`_.
About
This guide is a sister to another guide I wrote a while back about how to use DomainKeys Identified Mail (DKIM) with Postfix on Debian, which can be read here - /2010/01/11/dkim-on-debian-with-postfix/.
DomainKeys is an older implementation than DKIM, DKIM is a merge of DomainKeys and Identified Mail. Both DomainKeys and DKIM are used so having both configured is a good idea.
Getting started
Lets start off by installing the dk-filter
sudo apt-get install dk-filter
Once installed you can can create a public and private key set using the commands below, if you’re already using DKIM you can skip this step and just use your already existing key.
openssl genrsa -out private.key 1024
openssl rsa -in private.key -out public.key -pubout -outform PEM
sudo mkdir /etc/mail
sudo …