The POP Protocol (Part 3):
The STAT command and the LIST command are used to inquire about the messages stored in the mailbox. STAT shows that there are four messages with a combined length of 8184 bytes. The LIST command shows the size of each individual message in the mailbox. When it comes to mail, size matters because the system downloading the mail needs to know it has sufficient disk space to store it. In the previous listing, all of the mail messages are small, so storage is not an issue.
The first message from the mailbox is downloaded with the RETR 1 command. It is then removed from the server mailbox with the DELE 1 command. Normally, messages are retrieved in order, and are deleted after they are retrieved, but they don't have to be. When using telnet to input the POP commands, you're in complete control. You can download messages out of order, you don't need to delete the messages you download, and you don't need to download messages you delete. Deleting messages instead of downloading them is often very useful. On occasion, a corrupted or overly large message stored on the server causes download problems for the desktop client. From the client, the user can log on via telnet, and delete the offending message to get everything running normally again.
Of course, a POP connection is not normally run manually over a telnet connection. This is done here only to illustrate the function of the protocol. Only telnet to the POP port for testing. Using your knowledge of the protocol and the configuration, you can telnet to the POP port, and test whether your server responds. The telnet test proves that the daemon is available, installed, and ready to run.