Programming Internet Email

Using POP

POP3 provides a standard mechanism for retrieving electronic mail from a remote server. The messages stored on the server may be deleted (as is usually the case) or retained. This allows many millions of home Internet users to have electronic mail sent to a mail server at their Internet service provider, then retrieve it when they are online. Small offices can use POP in the same manner to pull mail for many users from a mail server at a home office or an Internet service provider. These remote mail servers are known as maildrops. In short, any user that wishes to receive email on his own machine or LAN, but does not have a permanent Internet connection, may use POP.

A maildrop host must provide several services. It must include an SMTP MTA, providing mail services for all local users (if any) as well as for users for which it acts as a maildrop. The MTA must have access to at least one MDA for local message delivery. Chapter 1 described the relationship of MTAs and MDAs. In order for this machine to provide services as a POP maildrop, it must also include an MRA for POP services. This MRA would consist of a POP server capable of answering POP requests from authorized remote clients.

A POP server is a TCP application daemon that listens for POP requests on TCP port 110.^[15]This is called a well-known port. Any POP client wishing to request service from a known host connects to this port. A server typically responds by spawning a new thread or a copy of itself (a fork) to ...