This thread may be a little old but still may help someone find a solution to this problem.
There is a plugin called 'Agent ticket-reply via email' for osTicket Motherload that is designed to do just that.
It allows agents to reply to tickets and can even auto-assign the ticket to the first agent that replies via email.
The plugin is free (but requires Motherload) and you can download it from the Motherload product page here:
https://www.cartmega.com/motherload-for-osticket.html
Benefits:
- Automatically assign the ticket to the first agent that replies via email (if option is enabled)! 👍
- Agents & customers able to communicate seamlessly via email without ever logging in to osTicket! 👍
- Never have to manually edit your class files again because you upgraded osTicket! 👍