POP and Server
POP is suitable for offline models of C / S structures. Offline models cannot be operated online, unlike IMAP4 (Netscape Support IMAP4) When the client is connected and queried and query new email, all the mail specified by the client will be downloaded to the client, After downloading, the email client can delete or modify any email without further interacting with the email server. POP3 customers send commands to the POP3 server and wait for a response, and the POP3 command uses the command line to reply with the ASCII code.
Server response is composed of a separate command line, or multiple command lines, responding to the first line with ASCII text + OK or -ERR to point out the corresponding operation status is successful or failed in the POP3 protocol There are three states, accredited status, processing status, and update status.
POP3 Service Principle
When the client is connected to the server, once the client provides its own identity and successfully confirmed, that is, the recognized state transfer is processed,
After completing the corresponding operation, the client issues a Quit command, then enters the update state, and then returns the recognition state after the update. As shown below
Wait for connection identification Quit Command
- | Approve | ----- | Process | ------ | Update |
| _______________________________________________________________________________ _ _ _ _ _ _,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,, -------------------------------------
userusername approved this command with the following If the pass command is successful, it will lead to state conversion
Passpassword recognition
apopName, Digest approved Digest is MD5 message Summary
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statnone processing Request server sent back to mailbox Statistics, such as the total number of mails and total bytes
UIDL [MSG #] to process the unique identifier of the mail, each identifier of the POP3 session will be unique
List [MSG #] Processing Returns the number of emails and the size of each message
retr [MSG #] processing returns all text
dele [msg #] processing by the message identified by the parameter The server will mark the message marked by the parameter identity as delete, and the Quit Command performs the
Rsetnone processing server resets messages that are marked as deleted, used to undo the dele command
TOP [MSG # ] The processing server will return to the N-line content before the mail identified by the parameter, and n must be a positive integer
NOopnone processing server returns a positive response
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quitnone update