This is the ONLY reason I am still using ZenDesk and now FreshDesk. Everything else you do is better, except this one thing.
Basically, if I respond to a user in email and CC a coworker, I need the user's response to also be cc'd to my coworker. We have several agents who deal with tickets ONLY in email, they never log into the interface. If they CC someone, it needs to become a permanent CC on the ticket, just like if a CC is added in the interface.
In ZenDesk, we can manage all tickets through email alone, and everyone stays informed about what's happening. I can set a CC in the automation rules if I want, and once a CC is added in email OR in the client, it stays until someone removes it. Without this one feature, we can't use FreshDesk, and I really wish we could. Every single other thing is an improvement.

