DKIM is a method enabling your receivers to make sure the message they received is coming from your own domain name.
This tool will generate the public / private 1024 bit key pair you will need to setup DomainKeys/DKIM. You must specify a domain and a selector. The domain and the selector are not used in the generation of the public / private key pair. They will only be used to provide server and DNS setup instructions specific to you.
DomainKeys validate that the domain of the from address matches the domain that is sending the message. DomainKeys do not validate the domain of the return path. You must specify the domain that you will use in the from address (From: header) of your messages. (If you will be sending out mail from multiple domains, you will need to go through this process for each one.)