First of all we install postfix package (if not already installed):

One of our Zimbra Collaboration Suite deployments stopped working all in a sudden: unable to login with given credentials. My first thought: “oh my, they hacked us!”.

Setup Zimbra Collaboration Suite on a cloud server without public interface (read: behind NAT) may waste you an afternoon trying to figure out why local emails (same domain) are not delivered, while sending/receiving mail from/to the internet works flawless.