A build material container outlet structure, and a build material nozzle structure is described. The build material container outlet structure comprises a connector to releasably connect to a nozzle structure of an external aspiration system. The outlet structure comprises an aspiration channel to aspire build material from a build material container to the nozzle structure of the external aspiration system. The outlet structure comprises a protrusion protruding from an inner wall of the aspiration channel, to extend into an end portion of a tube of the nozzle structure that is inserted into the aspiration channel and retain a valve element of the nozzle structure in an open configuration.