email reply-to not working correctly

I have a client who usually gets a booking notification on his mail and then he pressed “reply” and then that customer’s email is chosen as the receiver of the outgoing mail. The problem is that when he presses reply now, then his own email is chosen as the receiver. So I’ve had to go into the plugin and make it so that the customer’s email is included somewhere in the mail, so he can copy and paste it into the receiving field, as a quick fix. I hope this can be fixed