Email: POP & IMAP

Let's now discuss the other side of the coin for how email works.

We'll cover the following

POP and IMAP are used to retrieve email from an email server. Either one can be used. Let’s discuss both.


The most commonly used version of the Post Office Protocol (POP) is version 3, or POP3. This is how it works:

POP Phases

Emails are simply downloaded from the server in 4 phases: connect, authorize, transaction, update.

  1. Connect: The user agent first connects to the POP3 server on TCP using port 110.
  2. Authorize: The user agent authenticates the user with a username and a password.
  3. Transaction: The user can now retrieve emails and mark emails for deletion.
  4. Update: After the user agent quits and closes the POP3 session, the server makes updates based on the user’s commands. So if the user marked an email for deletion, it will delete it. No copy of a deleted email is kept on the server.
    • Note that only what’s in the user’s inbox is downloaded. Other folders such as sent items, outbox, or drafts are not synced. So POP3 does not synchronize the folders.

POP Modes

POP works in two modes.

  • Download and delete: Once emails are downloaded from the server to the user agent, they are all deleted from there.
  • Download and keep: Emails are not deleted from the server once they are downloaded onto the user agent.

However, with the download and delete model, you can only use one client to check your emails. If you use multiple devices to check your email, this method is not appropriate because emails will not look the same across devices at different times. Also, users won’t be able to reread emails from different devices.

Have a look at the following slides for an example of how emails might not be in sync on multiple devices with POP.

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