DomainKeys Identified Mail (DKIM) is the most complicated email authentication protocol out there. Even if you do extensive research, it’s hard to find one place that describes how it all works.
You don't configure your email client (Apple Mail) you configure your SMTP or PHPMail server which assumes you have your own server and domain name because DomainKeys Identified Mail (dkim) resides at the email server level. This article gives an overview of why and how to configure your server.
If we knew what it was we were doing, it wouldn't be called research, would it?
Of course your iCloud/.me/.Mac email account is hosted somewhere, but I think in this case you will probably need to contact Apple Tech Support. By-the-way, I would be very curious to know what they say.
If we knew what it was we were doing, it wouldn't be called research, would it?