The standard for signing and authenticating email messages has recently been trending away from DomainKeys and moving toward it’s successor, DKIM, or DomainKeys Identified Mail. DKIM has a lot of advantages over the old DomainKeys, such as additional signing algorithms and the ability for third parties to sign messages.
In order to keep up with the transition, we recently deployed a few improvements. First, new users will have the DomainKeys app disabled by default, and will instead use DKIM. Existing users will have their current settings maintained with the option to disable the DomainKeys app and enable the new DKIM app, but we recommend all users disable the DomainKeys app and let DKIM handle all your domain authentication needs going forward. Our support team is happy to help with the transition.
Switching from DomainKeys to DKIM will not have any impact on your deliverabilty. DKIM has been implemented by all the major email providers such as Yahoo and Google.
Let us know if you have any questions!
Recently? DomainKeys was deprecated in favor of DKIM in 2007.
DKIM has been around for a while, but DomainKeys has been and still is widely used. Google Apps didn't implement DKIM signing until January of this year. Gmail and Yahoo didn't check DKIM signatures until 2008, and they still check DomainKeys. The IETF did replace DomainKeys as a specification in 2007, but declaring new standards and implementing new standards are different things… the <center> tag in HTML has deprecated since the late 90s but that doesn't mean it won't work or isn't still widely used.
You should link to this post from the DomainKeys and Apps page. That would likely help with people who are trying to determine which apps they want / should use. I only found this because I was researching multiple domain white labeling.