[otrs] OTRS not using references header

Hi, This is probably right out of the FAQ, but I couldnt find it there. I am trying to get OTRS use the References email header to thread messages. As far as I can see, it seems to be using the Ticket number in the Subject line to thread the messages. Is there some configuration change required to enable header based threading instead of subject based threading? Thanks in advance for any pointers you can provide!