Actually, I think Apple would be a good fit. They already have a server OS that is built, just as Zimbra is, with open source components (samba, openldap, postfix, courier, spamassassin, wsftp, apache, etc.). The email server that is currently built into OS X Server is, well, un-Apple-ish. It's barebones, and they could benefit from having a system that is more polished, like Zimbra. As long as they pledge to maintain it's platform compatibility and offer it as a separate product (and continue to participate in its open source community, as they do with WebKit/Samba/etc.), I think they'd be a decent fit.
mark daniel | actc
zcs 6 ose | mac pro | mac os x server 10.4.11