A build material container outlet structure (100) comprises: a connector (107) comprising an interface surface (116) and an adaptor to receive a nozzle structure (202) of an external aspiration system; and a retaining structure (128) to releasably hold the nozzle structure in an engagement position with the interface surface of the connector; wherein the retaining structure comprises one or more retaining elements (118-1, 118-2, 118-3) disposed in the connector to exert an attractive magnetic biasing force on the nozzle structure of the external aspiration system to releasably hold the nozzle structure in the engagement position with the interface surface of the connector.