POP3 only specifies a mechanism for downloading messages. It does not provide a method for sending mail back to a server. SMTP is used for sending mail, while POP3 is used for retrieving mail. Nor does it define message content or formatting. Those issues are dealt with by RFC 822. For more information about this subject, refer to Appendix E: Message Envelope, Header, Body and Parts.
POP3 is an "off-line" protocol. Electronic mail messages are downloaded and read "off-line" by the user using their local mail application. The user has to download the mail and then disconnect from the server before they can read their mail. Any responses to be sent require another connection.
All mailbox and message management is also done by the client. If a user wants to maintain message folders, then they must copy the message to a folder on the local system. There are no folder management routines for POP3 servers. If you use multiple POP3 clients to read your mail, then these mailboxes will eventually get out-of-synch.
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