SMTP is only useful as a transport protocol between mail servers. It does not deliver mail to a user's mailbox, nor allow remote users to retrieve mail from the server. Local delivery is a function of the mail system in use. For example, cc:Mail uses a disk-based postoffice to store mail for each individual user on that system. An SMTP server for cc:Mail would accept messages on behalf of the cc:Mail users, but would not necessarily deliver that mail to the user. The cc:Mail postoffice server would likely handle the local delivery independently of the SMTP gateway.
Likewise, SENDMAIL and derivatives (including Unoverica Message Transport) have separate processes for the SMTP server and the local delivery. Incoming messages are stored in an incoming queue directory (SYS:/USR/LIB/MQUEUE by default), and then another process moves the mail from the queue directory to the end-user's mailbox.
However, cc:Mail, SENDMAIL and other systems are based on the assumption that the mail users are directly attached to the mail server, and able to read mail directly from the server's filesystem. This assumption is not always valid, as many times a user is on a far off system. Additionally, these remote users may not be using systems that are capable of hosting a complete electronic mail postoffice or SMTP server.
POP3 provides a solution to these problems by implementing a mail-retrieval protocol for electronic mail postoffices. Similar to the way that SMTP provides a protocol for putting mail in to a postoffice, POP3 provides a protocol for getting mail out of a postoffice.
The POP3 protocol therefore carries several assumptions. Chief among these is the fact that the users mail is kept on a reliable system accessible over TCP/IP. The user should be able to connect to the mail server, provide a username and password, and retrieve any mail that is waiting for them.
Several versions of POP have been developed. The latest and most popular version is POP3, and is defined in RFC 1939. POP3 servers are required to listen on TCP port 110, and this port is now reserved as a well-known service port exclusively for POP3's use. POP3 clients use any port number above 1024, just as any other TCP-based client application.
For more information about the POP3 protocol, refer to section D.3.1 POP3 Command/ Response Syntax. For more information about SMTP's restrictions, refer to section D.3.3 What POP3 Doesn't Do. For more information about TCP, refer to section B.5 Transport-Layer Protocols and Services.
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