Does this mean that the internal format of mailbox has to be standardized so any program on the client side can read the mailbox on any server?

Yes, for a client-side program to universally read mailboxes from different servers, the internal format of the mailbox must be standardized. This ensures compatibility between various email clients and servers, allowing seamless access and management of emails regardless of the underlying server technology. Standardization can help avoid fragmentation and improve interoperability across different platforms and services.