Great antispam idea
My idea is to integrate dkfilter into zimbra. You read the howto here
Imagine that zimbra has 100k users. Sending emails between them, yahoo and other server that have dkfilter will guarantee nospam.
Additionally zimbra as a company has power to be heard, this could be nice marketing feature, to say that zimbra integrated domain keys etc.
Of course this would mean that admins would have to add IN TXT records into dns, but some of them have the ability to do that.
You can always raise a bugzilla RFE request for Zimbra to consider.:)