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Many email providers ask me if I'm interested in using POP3, or IMAP. What exactly are these and when should I consider using them?

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IMAP is fantastic if you are using the same email account on multiple devices, such as a desktop PC, mobile phone, laptop and web interface. Whenever you read an email on one device, that is synced to the IMAP server and then synced down to all the other devices and marked as read on every device.

POP3, on the other hand, downloads the emails to a client then (typically) deletes it from the server. So this means if you get an email whilst you are out and about on your mobile phone and read it, you will not get it on your desktop PC.

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I'm a POP3 to IMAP convert, but IMAP is not without a few problems. Doesn't always work flawlessly... this seems to be largely dependent on which mail server you're connecting to and which client you're using. –  churnd Aug 24 '09 at 12:15
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You should notice too that usually you can configure your client to delete, mark as read, or do nothing with the emails that are downloaded via POP3. GMail lets you choose also to archive the mails downloaded (it's not common to configure those things at the webmail; you usually does it inside your client program). –  Igoru Aug 24 '09 at 12:19
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@Igoru: But note that POP3 is designed to have emails deleted, so if you're using POP3 and you're trying to sync like IMAP does, you're getting far worse performance than you would using IMAP, as POP3 is not designed to work like that. –  Josh Sep 21 '10 at 17:11
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POP3 and IMAP are two different protocols used for viewing mail. The big difference is that POP3 downloads the messages to your client. IMAP leaves the email on the server while there is a basic cache stored in a desktop client. IMAP has the reputation of being used with webmail providers. IMAP has better support for multiple clients since the messages are stored on the server. More information can be found in those two links.

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Here's a quick tutorial I wrote a few years ago on how email works, SMTP, POP3, IMAP, Webmail, ...

What is a Standard?

A standard is a set of rules that are followed by all developers around the world. Some standards include HTTP, SMTP, POP3, … There is official documentation that describes each individual standard and most standards have been around for 0 to 30 years.
Each standard document is a very detailed explanation of what the standard is and how it works. Typically a standard has an RFC number associated with it, but there are many different types of standards.

Protocols

SMTP and POP3 are ‘standards’. Each standard describes a different protocol. A protocol is any kind of communication between 2 or more computers.

What is SMTP?

SMTP is the ‘standards’ protocol that is used to send email. Your computer uses SMTP to send email. See RFC 821, August 1982

What is POP3?

POP3 is the ‘standards’ protocol that is used to receive email. Your computer uses POP3 to receive email. POP3 is also referred to as simply POP. See RFC 1939, May 1996.

POP3 typically will connect to the mail server and download messages to your computer. It can then optionally delete the message from the server (which it is usually setup to do).

How Email works

  1. User A wants to send an email to user B.
  2. User A writes up an email and presses send.
  3. User A’s computer, uses SMTP communication to send the email to User A’s (Yes A, not B) SMTP server.
  4. User A’s SMTP server, sends the email to user B’s SMTP server using SMTP communication.
  5. User B when he feels like it, contacts his SMTP server and uses POP3 to download the messages.

Some important notes:

The only way to send email is to use SMTP. (Actually you can also use MAPI and some other things but let's not get into that)

The only way to receive email is to use POP3. (Actually there is also IMAPv4, but we'll pretend that POP3 is the only way)

How Email Applications work:

SMTP communication is present on your computer, no matter what email client you use. Any time an email is sent out, your computer uses SMTP to send the email. It doesn’t matter if you're using Eudora, Outlook, Outlook Express, Mozilla Thunderbird, or a custom made program. All programs use SMTP to send emails.

By using standards you are guaranteed that, even know user A uses Outlook, and user B uses Eudora, and they both have different SMTP servers both of the users will be able to communicate.

What is HTTP?

Before I can get to what web mail is, you first need to know what HTTP is. HTTP is just another standard protocol. But HTTP is meant to download files and web pages, unlike SMTP which is meant to send emails. See HTTP 1.1 RFC 2616, June 1999.

What is web mail?

Web mail is an online web page that allows you to send and receive emails using HTTP.

But wait a minute, didn’t I just say that the ONLY way to send email was using SMTP?

Yes! What the web page does, is provide you with a form that you fill out. Your computer doesn’t know that it is any different from a form that you fill out to enter your credit card information, or a form that you fill out to enter your home address, or a form that you fill out to sign into another web site. All your computer knows is that you are filling out a form.

When you press the send button, your web browser sends the form to the server. The server knows that this form is for email though. So the server interprets the form and extracts the needed information. The HTTP server then uses SMTP to send the message. Because the only way that a message is going to get from User A to User B is using SMTP.

What the web browser has done is fooled you into thinking that you are sending an email. But what’s really happening, is that your web browser is filling out a form, and then the web server is using SMTP to send your email.

Can you give me a web mail walk through ?

  1. User A wants to send an email to User B, User A is going to use web mail.
  2. User A uses his browser to type in an internet address (for example: www.hotmail.com).
  3. User A’s computer uses HTTP to contact the server and ask for the web page that is used for web mail in this case.
  4. The server responds (using HTTP) to User A’s computer with a web page that gives him options to compose mail, check mail, …
  5. User A clicks on the compose a message link. Again User A’s computer uses HTTP to contact the server.
  6. The server responds (using HTTP) to User A’s computer with the web page (which contains a form) that allows User A to compose a message.
  7. User A fills in the web page and presses send. The page is sent back to the server using HTTP.
  8. In the background, unknown to User A, the web server uses SMTP to send the email to User B. Why? Because the only way to send an email is to use SMTP
  9. The server responds (using HTTP) to User A’s computer with a web page that says the email was sent.

How does the web server use SMTP?

Since SMTP is a standard protocol it uses SMTP in the same way any program would use SMTP. See the section ‘How email works’.

What is IMAPv4?

I mentioned IMAPv4 earlier. IMAPv4 is a second method used by email clients to retrieve your emails. IMAPv4 is also referred to as more simply IMAP. IMAPv4 is more complex than POP3, but gives you the ability to work on your email from multiple computers. If you use more than one computer, and you'd like to access your email from both computers, IMAP is the way to go.

IMAP stores all of its data on the mail server. In that way each mail client from each different computer can be in sync. When you read an email from one computer, your work computer will also see that the message is read. Since data is stored on the server, IMAP email accounts are typically more expensive.

Reference

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A bit too much "pasting the whole tutorial". Maybe you could have copied only the parts matching to the question. –  Gnoupi Aug 24 '09 at 13:26
    
@Gnoupi: I think it is all applicable and on topic, and written by me. –  Brian R. Bondy Aug 28 '09 at 18:24
    
It is all arguably on topic, but a link with some supporting text might be more appropriate, or a more focused answer. –  Slartibartfast Jul 6 '11 at 3:40
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Here is a good comparison of the two

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Can you include more of an answer? Perhaps copy/paste some of the details from your link. –  Jonathan Sampson Jul 15 '09 at 12:52
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In my opinion, why duplicate it. Just follow the link. –  Simon P Stevens Jul 15 '09 at 12:56
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Because posting the content (at least partially) makes Superuser rank higher for that item. Plus, it keeps the users from having to click your link to really know what is there. Give us a preview at the very least, please. –  Jonathan Sampson Jul 15 '09 at 13:40
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I agree with Jonathan - it also helps prevent link rot. I'd like to thing that Super User et. al. will be around for a long time, but we can't always say that about what someone links to (Wikipedia is probably an exception). –  LeopardSkinPillBoxHat Jul 16 '09 at 4:09
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Both are Internet standard protocols (RFC 1064 IMAP4 - RFC 1939 POP3) for retrieving email from a mail server. Most email clients implement these protocols.

IMAP ("Internet Message Access Protocol") allows you to organize your emails in folders* on the server which can by synchronized with your email client. IMAP also synchronizes the "read" flag and other properties of an email (if it was a reply, a forward etc.)

POP3 ("Post Office Protocol") emails are downloaded from a single inbox and are normally deleted after the download depending on your client settings.

You can't use IMAP or POP3 for sending email, that's what SMTP is for.

* Folders are called "mailboxes" in the IMAP specification.

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POP3 (Post Office Protocol version 3) is a protocol used to retrieve email from the server. POP3 protocol was created because of the design of the electronic mail system that requires the server to accommodate email for a while until it’s taken by an eligible recipient.

IMAP (Internet Message Access Protocol) is a standard protocol for accessing / retrieving e-mail from the server. IMAP allows users to choose e-mail message that he will take, make a folder on the server, search for specific e-mail messages, even deleted e-mail messages on server. This ability is much better than POP (Post Office Protocol) which only allows to retrieve/ download all the messages without exception.

SMTP (Simple Mail Transfer Protocol) is a commonly used protocol for sending electronic mail on the Internet. This protocol is used to transmit data from computer sending email to the recipient mail server.

For reference, you could read this http://signinx.com/difference-between-pop3-imap-smtp/

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If you use e-mail from a provider, it would be best to consolidate it intoa Gmail account, to make sure you keep control over your own e-mail address. So many people that switch providers feel the pain of losing their e-mail address, and there's totally no need for that.

Just sign up for Gmail (or any other non-provider e-mail account that allows you to fetch with POP3), and follow the instructions onsite on how to enable Gmail to fetch POP3 from your provider.

Then, access Gmail (with your provider mail inside) using web, POP3 or IMAP as you see fit.

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They're methods for retrieving email from a server. There are some nice comparisons, especially this one from UOregon. As others have said, the big difference is server-side versus client-side storage.

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I kind of think that POP3 stores massive amounts of emails server side.. maybe that not being so was an old limitation. Yahoo used to offer POP access free(don't know about now).. but one can have mail of unlimited total size with them.. and I can access email from that email account, via POP. And "leave it on the server", as OE allows as an option when reading via POP3. –  barlop Sep 15 '10 at 12:40
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