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The Post Office Protocol (POP)

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��ࡱ�>�� 9;����8�������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������5@ ��0�*bjbj�2�2 (>�X�X �������������������8 <��v\\\\\\\\$Ra�>�\\\\\>��\\S���\�\�\�\������\P rE�]���\d�i0�� � ������� �� \\�\\\\\>>�Introduction The Post Office Protocol (POP) was first issued on October 1984. The protocol suggests a simple method for workstations to access mail from a mailbox server. The protocol is dependent upon TCP (Transfer Control Protocol) and assumes that mail is posted by SMTP. POP in more detail, is a way for a server machine to store and serve mail for various client machines that are not connected to the Internet 24 hours a day. This makes it like an electronic Post Office Box, where your email is held in the POP server until you login and retrieve it. For example, if somebody sends you e-mail it usually cannot be delivered directly to your computer unless it is permanently connected to the Internet and has its own mail delivery system. The messages have to be stored somewhere. An Internet Service Provider is on-line 24 hours on 7 days of the week and can do that job. This is where the message will stay until either you retrieve it or the Internet Service Provider administrator (ISP) finds a pile of large storage files from messages. The ISP then deletes them because you have gone past your limit of space available for incoming messages and cannot receive any more. ...read more.


1. The Authorisation Stage (This how POP3 handles Authentication) Once the client has opened the TCP connection and the server has issued a greeting message starting with a positive response (+OK). Then the client has two possibilities. Either sending a user name and a password by USER and PASS commands or sending an encrypted password by the APOP command, as PASS does not hide the password on the screen. After verifying the identity of the client, the server acquires an exclusive access lock on the mail drop. If the lock isn't acquired the server responds with a negative status and may close the connection or stay in the authorisation stage in order to allow issuing new authentication commands. After the lock is acquired the server assigns each message, a message number starting with 1. 2. The Transaction Stage After the client has successfully identified itself to the server, the server enters the transaction stage. Tthe client may issue commands and get responses from the server to each command. When the client issues a QUIT command the server enters the Update stage. Some Commands in the Transaction Stage � STAT � How many messages in the mailbox. � LIST [msg] � List a particular message. ...read more.


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