I have two Zimbra servers running - in both cases, there is an MX record and A name record in DNS for the two domains (although a reverse-lookup on IP address for one of the servers gives the machinename.hostingProviderDomain.net rather than the machine's hostname).
Sending and receiving mail is fine for both (albeit I'm having unrelated issues with logger, av & antispam on the 64-bit OS).
But - mail sent from either domain to a Yahoo! email address (any address of the form @yahoo.com) ends up in that user's bulk-mail folder, until they open one up and click "Not Spam" in Yahoo.
Digging deeper into Yahoo's treatment / categorization of spam, they have implemented a digital signature system, called "DomainKeys", and mail that isn't signed using this public/private key system is marked as spam. They list postfix as being one of the mail apps having an implementation of DomainKeys, and give the URL http://jason.long.name/dkfilter/ for more info.
Soo... has anybody else experienced Yahoo! putting mails from a zimbra server into bulk mail?