Sagawa Express said personal information that may have been leaked outside the company after unauthorized access could amount to as many as 53.23 million records, including the names and addresses of people who sent and received parcels.
The courier had already announced that personal information might have been taken in the breach. The figure disclosed now covers sender and recipient names, addresses and other data tied to deliveries.
Delivery firms hold sender and recipient details for parcels moving across the country, which puts a breach at a major courier at the level of national infrastructure rather than a single company's customer list.
Sagawa Express has not said how the unauthorized access occurred or when it was detected, according to the information released.