This is off-topic for Zimbra, but a lot of you administer systems, and I, at least, had not heard of this until I stumbled across it today. Apparently there's a basic flaw in the architecture of DNS servers -- ALL DNS servers, regardless if they're Windows, Linux, whatever -- that invites poisoning of the DNS cache. While there has not yet been any known exploit of this vulnerability, it's likely it'll happen fast now that the flaw is public, and it's a goldmine for phishers and other identity thieves.
Windows Update includes fixes for your Microsoft servers, most Linux publishers include information for fixes for bind and bind9, so the ability to patch your own systems is out there and you should do it. Unfortunately, it's not just your servers you have to worry about, though; it's also your ISP's servers if you forward to them (which you probably do). So you need to verify if they have done their homework and patched theirs (hint, my AT&T forwarders are NOT patched).
Take a look at this article Opinion: Fix your flawed DNS ... NOW! and the vulnerability tester on this website DoxPara Research.
I don't like "the sky is falling" emails any more than the next guy. . .and the sky isn't falling yet, but you should really pay attention to this one, as you can be sure the Black Hats already are. . .