POP3 versus IMAP mail

Methods to check your mail:

Amplex supports several different ways to access your email:

  • POP3 (Post Office Protocol #3)
  • IMAP (Internet Message Application Protocol)
  • Webmail

The major difference between POP3 and IMAP is where the messages are stored.

When retrieving messages with POP3 the default behaviour is to:

  1. Retrieve from the mail server (at your ISP) the number of new messages on the server.
  2. Transfer the messages from the ISP to your computer
  3. Delete the messages from the mail server.

When checking a mailbox using IMAP a completely different thing happens:

  1. Compare the list of messages at the server and the local computer to determine message state (new, read, deleted, replied to, etc.)
  2. Show the current state of the mailbox.  Synchronize the state of the messages on the server and the local computer.

The big difference between the two is that POP3 REMOVES the messages from the server once it has transferred them to your local computer.  That POP3 removes the messages from the server is very important to understanding the difference between the two accounts.  IMAP leaves the messages on the server until they are deleted by you.
Webmail is simply a way of using a web browser to read your mail using IMAP.  Webmail interacts with your mailbox using IMAP.
Nearly all mail client software (Outlook, Outlook Express, Thunderbird, Incredimail, Entourage, Vista Mail, etc.) can be set up to check mail using either POP3 or IMAP but all default to POP3 unless told otherwise.

So why would you want to use POP3 or IMAP?   Which one should you choose?

If you always check your mail from the same computer then POP3 is a good choice.   Since POP3 transfers the mail to your computer you always have a copy of your mail and you can read it when you are not connected to the Internet.  Remember – POP3 will transfer the mail and then delete it from the server.   Once you retrieve your mail using POP3 it is erased from the ISP’s mail server.
If you check your mail from multiple computers then IMAP is a better method.   Since IMAP keeps the mail on the server along with the state of the mail (read, unread, replied to) it makes it much easier to check your mail from multiple computers.   If you have a computer at work and at home both set up to check the same account using IMAP you will see the same messages on both computers.   When you read a message on one computer and then check the other one the message will show up as having been read already.
If you set up two computers to check mail using POP3 then something really confusing happens.   If both computers are set to check mail every 10 minutes (the default) then the first computer to check after a new message arrives retrieves it and deletes it from the server.   Let’s say for example   your ‘home’ computer is checking for messages using POP3 at 5, 15, and 25 minutes after the hour.   Your ‘work’ computer is checking at 0, 10, and 20 minutes after the hour.   When a new message arrives at 2 minutes after the hour it will show up only on the home machine.   A message that arrives at 8 minutes after the hour ends up only on the work machine.   A message arriving at 12 minutes would only show up on the ‘home’ machine.   Very confusing if your at work waiting for a message to arrive.
Things can get very  confusing if you are using both IMAP and POP3 at the same time.  Keep in mind that Webmail is really an IMAP client.  Let’s assume your home computer is set up to use POP3 and you leave it running and it’s checking for new mail every 10 minutes.   If you’re at work and decide to check your mail using webmail you log in and don’t see any messages – because your home computer is retreiving and deleting the messages from the server every 10 minutes.  Or you get lucky and catch the message before your home computer retrieves it – and then you check again 15 minutes later and it’s gone -  because your home computer just retrieved it and deleted it off the server.

So what’s the moral of this story?

Pick a method of checking mail and stick with it – if you use webmail then always use webmail.

If you want to use both webmail and a mail program like Outlook then set it up to check mail using IMAP.

If you want to use POP3 to check your mail then make sure you DO NOT leave it running when you are not using it.

If your messages all suddenly disappear off webmail it’s a safe bet that somewhere you have a computer checking your mail using POP3 and all of your mail was transferred to that computer.

Are there exceptions to the above discussion?

Yes – there are options available in most mail clients to tell POP3 not to delete messages off the server, to delete them after a certain amount of time, or based on other criteria.  These options are available to make POP3 behave more like IMAP – but they are something of a kludge – your probably better off using a protocol like IMAP.

Two other things occasionally happen with mail:

When using POP3 if the connection to the server is interrupted before all of the messages are retrieved the next time you connect  you will get another copy of all the message you already received.   The is because messages are not deleted until after all the messages are transferred.
When using both POP3 and IMAP the POP3 client will occasionally show a message in your mailbox that says “DO NOT DELETE THIS MESSAGE – INTERNAL MESSAGE DATA”.   This message is stored on the mail server and contains information used by IMAP.   Occasionally a POP3 client accidentally retrieves this message.   You can safely delete the message without hurting anything.

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