Thanks for your prompt response.
Firstly, please find the below screenshot for our OSTicket tool version information.

Now coming to the issue, please find the below screenshot where an agent is added as collaborator.

Let me try to explain the scenario, This agent 'Devashish Shrivastava' has sent an email reply on the ongoing thread from outlook then in the OSTicket it added this agent as "collaborator".
Please let me know if you need any further information to resolve this. Additionally, is there a way to restrict system to create users form any particular domain like '@xyz.com' .