I have linux apache mysql and php server. My website uses the php mail() function and the server's postfix server to email other people.
I successfully installed SPF and DKIM on my server to reduce the likelihood of my website's email-outs from ending up in people's spam boxes.
In my research, I stumbled on Domain Keys which seems to be a "historical" version of DKIM. Do the big web mail services like Google, Yahoo and Hotmail still use Domain Keys? If not, then maybe I don't need to install it?