Pop3 allows users to fetch and download e-mails from a remote mailbox. does this mean that the internal format of the mailboxes has to be standardized so any pop3 program on the client side can read the mailbox on any mail server? discuss your answer.

Generally no. POP3 is an interface protocol. It describes how the data "goes over the wire", but not how it is stored. Concepts like sender, recipient, mail subject and body have to be available on the mailbox in some form, but as long as the POP service can extract or derive this information from the remote mailbox, possibly using some kind of conversion, the mailbox format is a private matter.

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