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How does IMAP differ from POP3 and what are the pros/cons of each and why choose one over another?

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up vote 14 down vote accepted

POP3 is an older protocol. It was designed to transfer messages to a client for reading; that client would be some other computer. Once the message was transferred, it would (usually) be deleted on the server, although there is an option to keep a copy on the server at all times.

IMAP is a newer protocol. It was designed to allow one or more clients to connect to a mailbox on a central server. The idea was that the server would continue to store and manage the email at all times, while allowing access from any client that requested it (with the proper credentials).

If you have a choice, generally speaking, IMAP will be superior for the following reasons:

  • you will not have to store nessages to your local machine, which means e-mail lost to a dying hard drive is no longer an issue, nor will it take precious space;
  • if you were using POP, once the email is transferred to your client, and unless you specify to leave a copy on the server, that client has the only copy. This exacerbates the first issue (e-mail lost to a dying hard drive);
  • downloads are done on-demand, instead of as a batch. Several large emails can take a significant amount of time in POP3, but in IMAP, you will only retrieve what you are attempting to read;
  • most IMAP implementations allow for multiple clients to connect to the same mailbox, but POP is typically a single-client-only process;
  • with IMAP you can organize your email into "folders" as you would on a local client; when you connect with a different client, you retain that structure, so all email clients see the same organization.

There are however times when POP3 is preferable, such as:

  • The email client you are using doesn't support IMAP (obviously you need to connect somehow);
  • You fully intend to transfer the message to a different computer and you do not wish to have a copy left on the originating server (such as when you are using fetchmail);
  • POP3 is generally supported more due to its age because there are so many implementations of POP clients out there, although IMAP is fast catching up;
  • your email server may impose quotas that do not make IMAP practical if you have a large amount of email you keep around
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They use different communications protocols and ports to communicate with the server. POP will always download the email to your client, although you can leave the copy on the server ususally as well, whereas IMAP can get just a listing of the emails and then give you any specific email when you want it so that essentially, the mail is always stored on the server rather than the client.

Depending on your email provider and the amount (and size) of email you receive, leaving the mail on the server could begin to get close to any quotas that are imposed; however, an advantages is that you can easily access the emails from different client machines if you have that sort of need.

This brief goes into a little more detail.

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An additional consideration is performance; use of IMAP becomes a lot less responsive over global connections, and actions like moving a mail between folders take a lot longer than POP (where moving a message is a purely local action)

If you're living in Australia and accessing a mail service (suh as google apps) in the US then this is a big concern, if your mail server is nearby it's a non-issue.

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