Zentyal community edition is one of the widely used application for managing infrastructure. Zentyal pushes frequent updates through Zentyal repositories.
I remember about one issue which was faced while trying to check the updates from Zentyal server and ended up receiving following error:
W: GPG error: http://archive.zentyal.org 3.5 Release: The following signatures couldn’t be verified because the public key is not available: NO_PUBKEY 8E9229F7E23F4777
On looking into the community forums it was observred that the problem is with the repository key which is used for pairing from Znetyal client where the installation lies to the Zentyal server providing the updated.
To fix the issue just add the Zentyal 3.5 key so that that connectivity could be established with repository with successful handshake. Please follow below steps:
For Zentyal version 3.5:
wget http://keys.zentyal.org/zentyal-3.5-archive.asc sudo apt-key add zentyal-3.5-archive.asc sudo apt-get update
For Zentyal version 3.4:
wget http://keys.zentyal.org/zentyal-3.4-archive.asc sudo apt-key add zentyal-3.4-archive.asc sudo apt-get update
Hope this helps!