Having the same issue here.
Zammad 3.4.x installed from RPM and running on CentOS 8.
Email channel works flawlessly. Fetchmail does a great job.
We, Agents, can see when a fellow Agent updates the ticket but End-Users/Clients do not get any notification, regardless the state of the ticket.
Ideally they (Clients) should get ticket updates when the status changes and, of course, when it is closed.
Is such functionality implemented? if so, how can we enable it?
TL;DR: The customer is not supposed to have notification settings. These are for agents only.
(I’ll continue below)
The right way, like you thought later on, is to use triggers.
This has several benefits over the normal notification settings for agents:
You decide if, what and how your customer receives information on ticket creations or even updates
the mail from is always the mail address of the affected group, this way you don’t use the notification sender which allows using multiple different brands within one system for example
Personally I would also add the condition “action is updated” to above example of your trigger.
It will trigger on every update of the ticket as long as the state is “closed”.
Personally and maybe also from a support and customer happiness view, I prefer “personal” responses over automated ones. If you provide me a solution via mail or by phone (because somehow you have to solve my problem ) I’ll know the issue has been solved. But that’s taste.
In order for triggers to actually send out mails, your group has to have a email address configured.
If it doesn’t, you can’t reply on tickets via mail and the trigger will not fire. There’s no fallback to the default notification sender.
After that you can select the email address in question to start.
The group creation is indepentend of that step because you might want to create a group without mail address or already have an account configured.
) I’ll know the issue has been solved. But that’s taste. 
