I see Yahoo is one of the supporters of DKIM, so I am curious if Zimbra's plans for DKIM-enabling ZCS are now on the fast-track?

Bug #11524 indicates Zimbra would need to wait for Amavis 2.6 series for DKIM support (I added myself to this bug), which is currently in pre-release.

I see in the Amavis 2.6 release notes that per-host, as well as per-domain, key signing is possible, and I would encourage Zimbra to adopt that feature to simplify things for those of us hosting multiple domains on a Zimbra host.