On Helix and the HPSCS PostOffice servers, mail bound for a user is stored in a flat text file. When the dovecot server software is invoked, it makes a temporary copy of that file, adding X-UIDL: headers by which individual messages may be quickly identified by eudora, and then downloads new messages to the desktop. When the download is finished, the entire file is copied back to its original location, and the transaction is complete.

From the client's perspective, the operating mode is "set and forget." Typically the client runs continuously, checking for new mail at regular intervals and triggering an alarm or other notification when new mail arrives. New messages are displayed in the client InBox, where the user handles them individually. However, it is the downloaded copies of the messages that are being manipulated. The server has no knowledge of what happens to a given message on the desktop.

Observations:

1. The user does not see any of the server's old messages, only those not previously downloaded to a particular desktop host; in fact, the only way to view or access old server-side messages is to remove the desktop client's cache of the articles it's seen

2. By default, the server does not delete any of the old messages

3. When the client deletes a message, it remains on the server

4. As old, undeleted messages accumlate on the server, each POP3 transaction requires more time and disk space

5. Usually the client tries to hide signs of difficulty, offering diagnostic warnings only after repeated failures

Several approaches to minimize the stress placed on the server by a POP3 client are discussed in the POP Performance Page.