There are many different technical supports and methods to create a distributed e-mail system: POP (post office protocol), DMSP (Hierarchical E-mail System Protocol) and IMAP (Internet Information Access Protocol). Among them, the POP protocol is the earliest and therefore the easiest to understand. DMSP has good performance in supporting "connectionless" operation, but it is largely limited to a single application (PCmail). IMAP provides an extended set of POP and DMSP, and provides three support methods for remote mail access: offline, online and connectionless.
POP protocol supports "offline" mail processing. The specific process is: the mail is sent to the server, and the mail client calls the mail client program to connect to the server and download all unread mail. This offline access mode is a store-and-forward service, which sends mail from a mail server to a personal terminal machine, usually a PC or MAC. Once the mail is sent to a PC or MAC, the mail on the mail server will be deleted.
POP3 does not support the expansion of mail on the server, which is done by the more advanced IMAP4. POP3 uses TCP as the transport protocol.
Protocol architecture
POP3 is ASCII information sent between client and server. POP3 command summary:
Command description
User name
Pass the user password
Mail information on STAT server
The amount of information obtained by RETR
Number of messages DELEted by dele
Number of messages displayed in the list
Top< messageid & gt & ltnombredelings & gt print x lines of information (including protocol headers) from scratch.
Exit exit the POP3 server.
Optional POP3 command:
APOP domain name digest authorization status is valid;
Hot news n transaction status is valid;
UIDL[ message]
POP3 reply:
+OK.
hiccup