KevinTheJedi Not sure what you are asking here. The agent is sending a response through the ticket using POST REPLY. The response appears to send correctly in osticket, other than the error found when examining system logs, and the fact that no email ever gets to the user who opened the ticket.
Again, this error only comes up (and the email does not send) if that agent goes into a ticket that is not assigned to that agent, and sends an email. If they claim that case or assign it to themselves (which they can do as all three of our users are administrators), and then send the email through the ticket osticket, the email sends fine and no error in the system logs.
I tried upgrading to 1.18 to see if that made any difference, but the error is still occuring.