[otrs] Postmaster Filter Issue

Hi, I have set up a mail filter to ensure that mail matching specific Delivered-to and To headers is automatically moved to a specific queue by setting the X-OTRS-Queue header. However, this does not seem to work if I send mail to the OTRS system using a webmail client. The mail ends up in the Raw queue. With an Outlook-type client it works fine. What could be going wrong here? Regards, Tawanda