OMG! I have installed Zentyal 3.3 and when I run sudo apt-get update, It work fine but the end of the line, there was an error that
W: GPG error: http://archive.zentyal.org 3.3 Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 8E9229F7E23F4777
I have fixed this issue with below commands by adding “archive.zentyal.org".
sudo apt-key add zentyal-3.3-archive.asc
sudo apt-get update
Hope this will help you as well, if yes, please show up in comments but please don’t forget to change -3.3 with your Zentyal Version..