The Post Office
July 16, 2009
For the last couple of months, the mail has been moving too slow. Last month I called my credit card company, because the check I had written out more than a week before was not cashed. Today I had to call my bank, because the check I wrote out over a week ago was not received yet.
This is very upsetting. I am very old fashioned in that I like to do things by mail, but I don’t want to have my checks arriving late. How can the post office get away with this. When I called my bank this morning they told me that this has been an ongoing problem for a few months now. So if you are paying by mail, make sure you mail way in advance, and call to see if your check is received. They raise the price of stamps, and cut the quality of service - not right.
The Post Office Protocol has undergone several revisions. It is customary to append the version number to the protocol’s acronym. POP3 has made earlier versions of the protocol, informally called POP1 and POP2, obsolete. In contemporary usage, the term POP is almost always associated with the latest version.
The design of POP and its procedures supports end-users with temporary Internet connections, such as dial-up access, allowing these users to retrieve e-mail when connected and then to view and manipulate the retrieved messages when offline. Although most clients have an option to leave mail on server, e-mail clients using POP generally connect, retrieve all messages, store them on the user’s PC as new messages, delete them from the server, and then disconnect.
In contrast, the newer, more capable Internet Message Access Protocol (IMAP) supports both connected (online) and disconnected (offline) modes of operation. E-mail clients using IMAP generally leave messages on the server until the user explicitly deletes them. This and other aspects of IMAP operation allow multiple clients to access the same mailbox. Many e-mail clients support POP as well as IMAP to retrieve messages; however, fewer Internet Service Providers (ISPs) support IMAP.
Clients with a leave mail on server option generally use the POP3 UIDL (Unique IDentification Listing) command. Most POP3 commands identify specific messages by their ordinal number on the mail server. This creates a problem for a client intending to leave messages on the server, since these message numbers may change from one connection to the server to another. For example if a mailbox contained five messages, and a different client then deletes message #3, the next connecting user will find the last two messages’ numbers decremented by one. UIDL provides a mechanism to avoid these numbering issues by assigning a string of characters as a permanent and unique ID for the message. When a POP3-compatible e-mail client connects to the server, it can use the UIDL command to get the current mapping from these message IDs to the ordinal message numbers. The client can then use this mapping to determine which messages it has yet to download. IMAP uses a 32-bit unique identifier (UID) that is assigned to messages in ascending (although not necessarily consecutive) order as they are received. When retrieving new messages, an IMAP client requests the UIDs greater than the highest UID among all previously-retrieved messages, whereas a POP client must fetch the entire UIDL map. For large mailboxes, this can require significant processing.