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Authenticated Post Office Protocol (APOP) permits a client computer to retrieve email from a Post Office Protocol (POP) server while providing authentication technology that includes password encryption upon client receipt.
In standard POP, the username and password are in plain text whereas APOP encrypts them. The server that holds email is known as the POP server, so in APOP, clients leave email on servers in order to read them later. APOP does not provide services for sending email.
APOP is now an outdated method for mail authentication encryption and is not MIME-compatible. Newer technologies such as TLS have superseded APOP.
The POP3 is a one way only mail transport allowing a user to access mail and have it delivered over the internet. It is a receive-only service as far as the user is concerned. Simple Mail Transfer Protocol (SMTP) is generally used to send mail.
POP is primarily used for computers that don't have a fixed network connection and thus need a post-office type of setup to hold email until users are ready to retrieve it. APOP is an extension of POP that adds more security. With regular POP, usernames and passwords can easily be intercepted on a network creating a serious security vulnerability. To combat this issue, APOP was developed and used alongside other encryption technologies such as TLS and SSL.
APOP encrypts the password and username before they are transmitted and decrypts them upon receipt. This makes hacking into email much more difficult.
APOP is older and less useful than Internet Message Access Protocol (IMAP) and its biggest disadvantage over IMAP is that the APOP email retrieval process is more arduous and less specified, and emails are not automatically categorized into certain orders such as when they were received. This disadvantage especially pertains to users sifting through large amounts of email within APOP.