The DomiainKeys Identified Mail functionality provides an email authentication method to help ensure that a message is not modified while it is in transit and the receiver will still know that the email is coming from you. Watch the following video on how the DKIM Integration works.
Click on your Profile Icon then Click on "Account Settings"
Click on "Email Auth"
Click on "Add Domain"
Choose the domain that you would like to use for your emails. Click on "Next"
5. Add all the generated CNAME'S into your DNS account. You might have to wait a few minutes before you Click on "Validate". It depends on the TTL that is added within your DNS account. GoDaddy can be given a TTL as low as 600 seconds.