Post Office Protocol

POP stands for 'Post Office Protocol' and is the older of the two protocols. When a POP compliant email reader retrieves email with POP, the reader creates a connection between itself and the user's POP server. The POP server is running a piece of software called a 'POP daemon' which is listening for these connections. Once connected, the POP daemon will ask the reader to authenticate the user by asking for a username and password. Assuming both are correct, the POP daemon then sends a copy of each mail message on the POP server up to the reader. In most cases, the POP daemon then deletes the email on the POP server, but this is configurable by the user. The user's email is now stored locally on his or her PC.

IMAP stands for 'Internet Message Access Protocol' and is a fairly new development. Until recently, not many email readers supported this protocol. One of the first is the popular UNIX email reader 'pine'. The setup is basically the same as POP where we have a IMAP compliant email reader and a IMAP server running an IMAP daemon. Once connected, the IMAP reader will request a list of the available messages from the IMAP daemon. As the user chooses to read each message, the IMAP will upload a copy of the message to the IMAP reader. But, IMAP will not upload the entire mail file and then delete it like POP.

Thus here are the main differences between POP and IMAP. With IMAP all messages are kept on the IMAP server, even saved and read messages. With POP new, unread messages are stored on the POP server and old or saved messages are stored on the PC. What's the difference? Well with IMAP it is possible to read email from several different locations on the Internet and from more than one IMAP reader. Thus one can read email by day from work and then continue reading by night from home. Since the email folders are all stored on the server, this is easy. To duplicate this behavior with POP, one must configure the POP reader to not delete email on the POP server once it is uploaded. This can thus create several copies of the email on different PCs and can make things confusing.

Here is an edited list of IMAP features from the documentation on 'pine'. These items show some of the benefits of IMAP:

• Allows access to mail folders from more than one client computer.
• Works well over low-bandwidth lines because information is sent in small pieces as needed by the user. For example, only header information is sent to build index lists, and if someone sends a 2MB audio file via MIME, you can choose when (or if) you want to get that part of the message.
• Email can be delivered and stored on a well-maintained and reliable server which is "always-up".
• Folders can be accessed and manipulated from anywhere on the Internet.
• Users can get to messages stored in different folders within the same session.
• Allows use of IMAP server for searching and parsing.
• The latest revision of IMAP (IMAP4) also provides for disconnected operation, including resynchronization of message state between mail servers and message caches on clients. Pine does not yet support this capability, however.

Hopefully you have noticed the 'centralized' behavior of IMAP. For users who make heavy use of email from multiple locations, and IMAP reader will be much easier to use. However, we must point out that POP is not a bad or worn out protocol. POP still accomplishes its purpose and it does it well. Also, considering that most email readers are POP compliant and that very few are currently IMAP compliant, POP will be around for a long time. We at PRIMUS do not recommend one protocol over the other, but we are committed to providing a choice to our customers and we are always available to offer help and answer your questions regarding POP and IMAP.

For a more detailed discussion on POP vs IMAP, please check out the document "Comparing Two Approaches to Remote Mailbox Access: IMAP vs. POP" by Terry Gray. Copies of this document are available from ftp://ftp.cac.washington.edu.

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