Zammad actively tries to avoid to reply to agents and its own email addresses.
In most scenarious replying to an agent is not exactly “useful” because they (not always, but most of the time) have access to the information in Zammad already.
Especially, because you’d usually don’t want to “leak the agents email address” to a customer. Customers then tend to send to agents directly and your agents either don’t put it into Zammad or answer directly. That’s not exactly the idea you have when you implement a ticket system.
It’s usually supposed to be the single spot of communication. That may not apply to all use cases which is why you can do that manually. Maybe that view helps you.