We are currently working on implementing a direct track and trace link on package level on carrier PostNL. The link should also be possible to use with the mail back feature.
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Update: 06-09-2018, this has been released in Consignor On-premises
Update: 07-09-2018, this has now been released in Consignor Ticket
Update: 10-09-2018, this is now available in Consignor Shipment Server