When an assignment email is sent out to the Technician, and they reply back, it does not go to the Requestor. It is more of an information email and requestor would not be involved as to whom their ticket is assigned to. The requestor is emailed to via the ticket itself from an Update or Comment feature. Then, a technician can reply back if a requestor sends an email to the technician. That response appends back to the Feed of the ticket.
The email reply service is intended for outbound and inbound responses on the feed to the requestor or a contact record that is on that ticket.
Please let us know if you have any additional questions.