What is POP3 service?

POP3 (Post Office Protocol 3) Pop is an email protocol for offline models in the C/S structure,
It has now evolved to the third edition, POP3 ., It specifies how to connect a personal computer to an Internet Mail
Server and email download protocol. It is the first offline protocol standard for Internet email,
POP3 allows you to store emails on the server to a local host (that is, your computer) and delete the emails.
The mail is saved on the mail server, while the POP3 server is the receiving mail server that follows the POP3 protocol,
Used to receive emails.
Pop is applicable to offline models with a C/S structure. Offline models cannot be operated online, unlike IMAP4 (Netscape
Supports IMAP4) when the client connects to the server and queries new emails, all emails specified by the client
All downloaded emails will be Program Download to the client. After the download, the email client can be deleted or
Modify any email without further interaction with the email server. POP3 customers send messages to POP3 servers
Command and wait for response. The POP3 command uses the command line format and ASCII code.
A server response is composed of a single command line or multiple command lines. The first line of the response is in ASCII
Text + OK or-err indicates whether the operation is successful or fails.
Status, processing status, and update status.
When the client establishes a connection with the server, once the client provides its own identity and is successfully confirmed, it is recognized
Status transfer to processing status,
After the corresponding operation is completed, the client issues the quit command and enters the update status.
Status. For example

Wait for the quit command to confirm the connection identity
-- | Approval | ----- | processing | ------ | update |
| __________________________________ |
Return to approval status

POP3 command code:
command parameter status description
--------------------------------------------
If User Username accepts this command and the following
PASS command, this will cause status conversion
pass password approval
APOP name, digest recognizes that digest is an MD5 message
abstract
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the stat none Processing request server sends back the mailbox statistics about
, for example, the total number of mails and the total number of bytes
UIDL [MSG #] unique identifier for handling returned mails
identifier, each identifier of a POP3 session will be unique
list [MSG #] Number of returned mails processed and the size of each
email
RETR [MSG #] processed return all text of the
email identified by parameters
DELE [MSG #]. The processing server marks the email identified by the
parameter as deleted, run the quit command
rset none processing server will reset all emails marked as deleted
, the DELE command is used to undo the DELE Command
top [MSG #]. The processing server will return the first n lines of content of the email identified by the
parameter, N must be a positive integer
Noop none processing server returns a positive response
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quit none update