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So, you’ve got your nice little email program (such as OE Classic), and you want to configure an email account and it offers you a choice between POP (or POP3) and IMAP but you don’t know which one to choose. What is the difference?

First off, both POP and IMAP are email protocols designed to fetch emails from a server. As email is a two-part system, the first part being an email program which you install on your computer, tablet or smartphone, called the email client, and the second part being the email server.

Think of it as a postman and a post office.

The postman (email client) goes to the post office (email server) to fetch some emails. Post offices (email servers) communicate between themselves to send emails and finally a postman on the other end (another email client) fetches the mail from another post office. In reality, it is a bit more complicated than that, but that is the general idea.

To communicate, both the client and the server must agree on a common standard, or protocol – and POP and IMAP are the names of these protocols, and they represent a set of commands (or a vocabulary if you will), designed to make it possible for the client and the server to understand each other.

Now, do not be confused by the name POP3 as that is also just POP, just with the number 3, which indicates the version of the protocol (before POP3 there were POP1 and POP2 which are no longer in use today).

So let’s start with POP protocol.

POP, which stands for Post Office Protocol is the older and simpler of the two protocols and it is designed with a simple goal of fetching emails off of a server by a single email client and then removing them from the server. It is also possible to keep emails on the server but more on that later.

How does POP work?

1) First, an email client connects to the server and authenticates with the username and password you put into its account settings. At that moment, the mailbox on the server is locked and you cannot access it from an another computer or device – POP protocol specifies that another connection is disallowed, while the current one is in use. The lock is there to prevent 2 computers to access the mailbox at the same time as POP is not designed for that. (POP commands used: USER, PASS, APOP).

2) Then it examines the list of messages the server offers and figures out which ones are new. The server replies with the list of messages and their unique identifiers. POP can only check the contents of your Inbox folder messages and that is the only folder it can access usually. (POP commands used: STAT, LIST, UIDL, TOP).

3) After establishing the new messages list, email client proceeds with the download, one message at the time. (POP commands used: RETR, TOP).

4) At the end of the session, email client decides, based on the options you’ve set whether or not it should delete some emails. Deleting messages can happen immediately or after a few days, again depending on the options in your email client. When it is done, it disconnects. (POP commands used: DELE, QUIT).

5) Right after the client has disconnected, the server performs actual deletion of messages (as they cannot be deleted before the QUIT command is sent by the client) and frees the connection for another email client (or a repeated connection by the same email client).

All of the above happens very quickly due to POP protocol being very lightweight and simple.

How does IMAP work?

1) Just like with POP, an email client connects to the server and authenticates with your username and password. (IMAP commands used: LOGIN).

2) At this point an email client can choose which folder (also called mailbox) it wants to access and selects that mailbox. For example that folder might be the “Inbox”, “Junk”, “Trash” or any other folder available on the server. (IMAP commands used: SELECT).

3) Then, usually it fetches the list of messages in the selected folder and updates the local list it already has, removing the messages which are no longer present and adding the new ones to the local list. In other words, it synchronizes local list to match the server list. (IMAP commands used: FETCH).

4) Since IMAP is more complicated, now a few things can happen – an email program can continue checking other folders like “Sent”, “Deleted”, “Drafts”, “Junk” and so on, so it might return to the step 2 in this example. In that process it will again update local lists and synchronize it with the server message list. However, it can also choose to remain idle in the current folder and remain connected, waiting for new messages to arrive. You can also search for messages in folders as IMAP also offers search capability. (IMAP commands used: IDLE, SELECT, FETCH, SEARCH).

5) As IMAP is designed to be “always online” any messages you download are just stored temporarily, as a cache, to avoid downloading it again. If a message is deleted from the server, for example by another email program or device, the currently running program will simply update its cache to reflect the changes done on the server the next time it synchronizes the folder. Note that unlike POP, IMAP doesn’t prevent other clients to connect at the same time. Multiple connections may happen and they will eventually be synchronized between each other.

6) If you delete a message, the deletion happens immediately and the change is again synchronized with every other client or device connected at the same time. The same happens if another email client deletes a message – eventually it is synchronized to your email program and you see the change as well. Additionally, IMAP supports flags such as \Seen, \Draft, \Deleted, \Answered, \Flagged, common among email clients to indicate if the message is read, draft, deleted, replied and flagged (starred), respectively. (IMAP commands used: COPY, MOVE, STORE, EXPUNGE).

7) At a certain point you decide to disconnect from the server and your synchronization then stops and resumes the next time you connect to the server. If your email client has cached the messages they can be still read, while offline, but if the messages are not cached, an Internet connection will be required to read them. Also, if you decide to move message to another folder or delete them, an Internet connection will again be required as the change cannot be performed while being offline.

So unlike the POP protocol which is like fetch-and-delete kind of protocol better suited for offline use on a single device, IMAP is more like always-online protocol which just shows you a window into the messages stored on the server (or in the cloud, if you will).

What about advantages and disadvantages of both and how to choose one?

The choice really depends on your situation.

To put it simply:

• If your goal is just to download messages on a single computer and remove them from server, then POP is an easy choice, giving you just that. It is designed to be fast and simple for use on a single device.
• If you want to synchronize or use multiple devices or computers and you want to have identical copy of your email on each of those devices, including all of the folders and messages, then you can use IMAP instead.

Advantages of POP

– It is fast and lightweight and uses less bandwidth (may not be an issue nowadays with fast connections but might be an issue in a rural area or with a mobile connection). This also makes it easier choice when access to the Internet is not available all the time.

– It downloads messages from server onto your computer.

– You have a local copy of messages which means you can read them even if you are disconnected from the Internet.

– Emails are removed from the server so if your email account is hacked nobody can read your older emails as they are downloaded onto your computer. Note though that many email programs (including OE Classic), allow you to leave a copy of messages on the server and delete after a certain time has passed. Option like this can help you download email onto multiple computers.

– Organizing your email is much faster as they are basically just local files so there isn’t a need to communicate the change with the server. So it can be done while offline as well.

– If you lose access to your email account or your email account is hacked, your emails are stored on your disk drive and are not in the cloud (on the server).

– The size of the mailbox only depends on the size of your disk drive and you don’t have to pay for the extra space on the server. Also, if your server offers low storage capacity POP will help you keep that storage clean if you choose to delete messages from server after downloading them.

Disadvantages of POP

– Since it usually deletes messages from the server, you have to have a backup plan for your emails. If your disk drive fails for any reason, so will your messages as well as they are just files on the disk drive. Note that this applies to all of your data which you store on your disk drive, not just emails.

– It does not synchronize server folders and messages. The only folder it has access to is the “Inbox” folder but it is not synchronized. Even though many email clients (including OE Classic) allow you to leave a copy of messages on the server and with that option enabled it is possible to download emails onto multiple computers, when you want to delete a message from one computer it is not deleted from all of them so they are not synchronized. This is especially a problem if you want to use email from multiple devices like mobile phone, computer, tablet and want to have identical copy of all folders and messages across all of your devices.

– It only can access “Inbox” folder unlike IMAP, so if your emails end up in the server “Spam” folder you cannot see them – you have to use webmail or IMAP to fetch “Spam” folder messages as well. Although some POP servers do offer tricks to fetch messages from different folders, this is not really a rule and many of them don’t so the lack of access to the server folders makes it a disadvantage.

– If you lose your computer data, have your device stolen, broken etc., your emails will be lost with it unless you have a backup. The same goes if your email folders are damaged. Having a backup is important.

Advantages of IMAP

– If you use multiple computers or devices you can have your email folders and messages synchronized between these devices – if you delete a message, add or rename a folder, receive a message, change the message status from read to unread or add a flag – all of these changes are registered across all of the devices connected to that email account, when they synchronize. So you can access it on the go or in the office (or at home).

– Your messages are backed up in the cloud so aside from local cached copy there is also a cloud (server) copy. Servers, if properly maintained, are regularly backed up so your emails are safer that way.

– More people can use the same mailbox and organize/delete/flag messages. This makes it a better choice for single mailbox shared between team members, for example if a single company mailbox is answered by multiple employees.

Disadvantages of IMAP

– IMAP involves much more communication with the server so it will eat up more bandwidth and Internet. Also, as each change has to be registered (synchronized) with the server, it is slower, so it depends on your Internet speed and server speed. Servers which are unstable or slow might cause problems so the server has to be good as well.

– If your server is unreachable, hacked, or if you’re offline, you cannot read your email except the cached messages in your email client. If messages have not been cached, you won’t be able to read them. You also cannot organize your emails while offline – move, copy, flag, mark as read, all of these operations require to be synchronized with the server so you have to be online to perform them.

– If you lose access to your email account, you may also lose access to all of your messages, again, unless they have been cached locally.

– The size of your mailbox depends on the amount of disk space you have available on your email server and you may need to pay for the extra space if it is not sufficient.