[otrs] Ticket follow-up question

Hi all, We are starting to use OTRS in our business, And (of course) i have some questions........ How OTRS track a follow-up to a ticket ??? For example, if the agent reply (via e-mail) to a message WITHOUT leaving the ticket ID in the subject, the subsequent reply from the customer comes as a new ticket.......Here (in some cases) we dont like to answer with the ticket ID in the subject.... theres a "trick" to use a X-header for ticket tracking ??? example: OTRS can insert a x-header like: X-OTRS-TT: TICKET 1234567890 and then track the ticket using this X-header, without using the "subject" field..... Is this possible ??? Many many thanks Marco Cintolesi