Hi there. The Postmaster is not meant to be used as long-term storage. It has limited space and is meant to collect items not picked up when you go to Orbit or when you receive an activity reward while your inventory is full. Players are intended to dismantle or pick up those items to put in their inventory or vault. If there are more items sent to the Postmaster than space available, old items will be removed to make space for the new ones. It's been repeatedly recommended that players occasionally check and clear out their Postmaster to avoid losing things.