sis-queue: Parent filesystem not given as parameter

I tried to change: mail_attachment_fs = sis posix to mail_attachment_fs = sis-queue posix

and immediately failed with:

Failed to initialize user: Namespace '': mdbox: mail_attachment_fs: sis-queue: Parent filesystem not given as parameter

Where do I specify the "Parent filesystem"?

-- Ralf Hildebrandt Geschäftsbereich IT | Abteilung Netzwerk Charité - Universitätsmedizin Berlin Campus Benjamin Franklin Hindenburgdamm 30 | D-12203 Berlin Tel. +49 30 450 570 155 | Fax: +49 30 450 570 962 ralf.hildebrandt@charite.de | https://www.charite.de