what email template are you using? is this a custom one? Does it happens with a particular message? or it happens on any email your systems sends back to a user?
I'm using osTicket Default Template (HTML) (System Default). It happens on any e-mail.I'm doing it right after clean installation of 1.8.3. I didn't setup any signature. Staff panel -> My Profile -> Default Signature, right? I'ts - None -. "Signature" on the bottom is also empty.