POP3 or IMAP Explanation and troubleshooting

The main difference between the two protocols is the way email is managed locally (POP3) vs server side (IMAP).  We take a closer look below from How Stuff Works explanation.


In the simplest implementations of POP3, the server really does maintain a collection of text files -- one for each e-mail account. When a message arrives, the POP3 server simply appends it to the bottom of the recipient's file.

When you check your e-mail, your e-mail client connects to the POP3 server using port 993. The POP3 server requires an account name (email address) and a password (for that mailbox). Once you've logged in, the POP3 server opens your text file and allows you to access it. Like the SMTP server, the POP3 server understands a very simple set of text commands. Here are the most common commands:

  • USER - enter your user ID
  • PASS - enter your password
  • QUIT - quit the POP3 server
  • LIST - list the messages and their size
  • RETR - retrieve a message, pass it a message number
  • DELE - delete a message, pass it a message number
  • TOP - show the top x lines of a message, pass it a message number and the number of lines

Your e-mail client connects to the POP3 server and issues a series of commands to bring copies of your e-mail messages to your local machine. Generally, it will then delete the messages from the server (unless you've told the e-mail client not to).

You can see that the POP3 server simply acts as an interface between the e-mail client and the text file containing your messages. And again, you can see that the POP3 server is extremely simple. You can connect to it through telnet at port 110 and issue the commands yourself if you would like to (see How Web Servers Work for details on telnetting to servers).

 POP3 example


As you can see, the POP3 protocol is very simple. It allows you to have a collection of messages stored in a text file on the server. Your e-mail client (e.g. Outlook) can connect to your POP3 e-mail server and download the messages from the POP3 text file onto your PC. That is about all that you can do with POP3.

Many users want to do far more than that with their e-mail, and they want their e-mail to remain on the server. The main reason for keeping your e-mail on the server is to allow users to connect from a variety of machines. With POP3, once you download your e-mail it's stuck on the machine to which you downloaded it. If you want to read your e-mail both on your desktop machine and your laptop (depending on whether you're working in the office or on the road), POP3 makes life difficult.

IMAP (Internet Mail Access Protocol) is a more advanced protocol that solves these problems. With IMAP, your mail stays on the e-mail server. You can organize your mail into folders, and all the folders live on the server as well. When you search your e-mail, the search occurs on the server machine, rather than on your machine. This approach makes it extremely easy for you to access your e-mail from any machine, and regardless of which machine you use, you have access to all of your mail in all of your folders.


Issues related to POP3 configurations:

Mailprotector supports both protocols but with todays multi-device and folder management needs, typically an IMAP configuration works best.  If you previously used POP3 and want to continue to use POP3, that is fine but be aware of the limitations associated with this protocol and some of the side affects you may experience.

  • Missing email - When you download a message from the server with a typical email client, the default behavior is to remove the email from the server.  Once this has been performed, you will not be able to access the same email from another device (mobile, laptop, or other pc).
  • Duplicates - In a situation where your local pc see's a new identifier associated with the email, it will download the same message as new and create duplicates.
  • No server side backup -  Once the email is removed from the server, it exists on the local client/pc only.  If something happens to this computer, the data can be lost with no backup or way of retrieving it.  Once the delete command has been performed from the POP3 client, we no longer have a copy.
  • No webmail access - Webmail provides a web based view of everything on the server.  Once this message is removed (using the POP3 client), you will no longer have access through webmail.
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