Post Office Protocol (POP) enables a workstation to retrieve mail from a mail server. With POP, mail is downloaded from the server to the client and then deleted on the server.
The server starts the POP service by passively listening on TCP port 110 for client connection requests. When a client wants to make use of the service, it sends a request to establish a TCP connection with the server. When the connection is established, the POP server sends a greeting. The client and POP server then exchange commands and responses until the connection is closed or aborted.
Because email messages are downloaded to the client and removed from the server, there is not a centralized location where email messages are kept. Because POP does not store messages, it is undesirable for a small business that needs a centralized backup solution.
POP3 is desirable for an ISP, because it alleviates their responsibility for managing large amounts of storage for their email servers