[otrs] In-Reply-To header not set

Hi everybody We are using OTRS 2.3.4. Since our upgrade from 1.x the In-Reply-To Headers are not set in outgoing mails. Because of this threading is only possible with the subject as key. As it worked in 1.x I think this is a bug? I already tried to set "$Self->{PostmasterFollowUpSearchInReferences}", but this did not help. Any ideas? Do you need more informations? Regards raf