DKIM is a special digital signature affixed to outgoing emails in their headers. It permits any recipient of incoming mail to be sure that the letter was in fact sent from that domain. It is unlikely a DKIM-signed message will end up in a spam folder.
To turn DKIM on, you must add a special TXT record to your domain's DNS. It takes a day to get it up and running. As soon as it is up and running, all messages will begin to receive your digital signature automatically.
DKIM signature is available here, replace the demo value with your own username and domain name. (This above method has been deprecated, you'll now receive the DKIM signature on your registered email after verifying your domain)