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POP3 (Post Office Protocol) is a very common protocol for getting emails from a mail server over a glossary("TCP"} connection. POP3 does not support folders, unlike the more recent IMAP4, which is harder to implement because of its more complex structure.
Clients usually retrieve all messages and then delete them from the server, but POP3 does allow retaining a copy on the server. Nearly all email servers and clients currently support POP3.