IMAP or POP3 – comparing e-mail protocols

E-mails are transported via mail servers and are received with the help of e-mail programmes (also known as e-mail clients). These clients (e.g. Mozilla Thunderbird or Windows Live Mail) rely on protocols in order to display e-mails. The approach is different depending on which protocol is used: the IMAP network protocol opens the message directly on the server, while the transfer protocol POP3 ensures that the message is first downloaded and then opened locally.

The Internet Message Access Protocol, in short IMAP, is a text-based network protocol that enables access to e-mails located on a mail server. If you set up your account with IMAP, your e-mail client establishes a connection with the server every time you log in. This connection is maintained for the whole session. During this time you can access individual folders and e-mails whose content can be shown on request. This way all messages and folder structures stay saved on the server until they are deleted. As a result, they can be accessed from anywhere and with numerous clients and always find the same, current database.

The connection between the server and IMAP client is established via TCP/IP on port 143 (with a secure port 993 connection). The client communicates with the mail server via text messages, but doesn’t, however, need direct answers to the commands that have been sent out.  In order for the response from the server to be registered at a later stage, the client attaches an identifier to its commands, which the mail server adds to the response. The reply line starts with an asterisk when information is contained. If the reply contains a plus sign at the start, it means the server is expecting additional information about the received command. The response additionally provides information on the success (OK), failure (NO), and syntax errors of the particular IMAP client’s commands.

The Post Office Protocol (POP3) enables e-mails to be retrieved with the help of a client. For this purpose, the client establishes a connection with the incoming mail server on which the necessary POP3 server software must be installed. The e-mails located there are downloaded and saved onto the client’s computer. Following this,, the e-mails are deleted by the mail server and the connection is terminated. The e-mail content can be opened locally and edited without the client and server being connected. The duration of the retrieval process depends on the size of the mail content or attachment. Each message can only be downloaded by a single POP3 client.

POP3 clients rely on port 110 during the connection process to the mail server through TCP/IP. If the connection is encrypted, port 995 is used. When the server and client are connected, they communicate via commands. POP3 commands are made up of three to four characters and one or more parameters. The server answers each command directly with a positive (+OK) or a negative (-ERR) status message as well as some optional information. Each POP3 session is divided into three steps: The first step is user registration where the client enters their user name and password via the mail server. Next comes the actual process of retrieving the message. When all mails have been downloaded and successfully separated from the client, they will then be deleted from the server as part of the last step. If there’s a connection failure during the retrieval, all the e-mails will remain so that the POP3 client can continue to download them once the connection has be re-established.

A comparison of both protocols shows that there are some elementary differences between IMAP and POP3: whereas clients with IMAP establish a permanent connection to the mail server, the POP3 client and POP3 server are only connected when an e-mail is obtained. The e-mails are also handled in different ways: with POP3 the downloaded e-mails are deleted by the mail server. If they rely on the services of the IMAP network protocol, all messages stay on the server until they are manually deleted. That is also the reason why numerous clients can have access to the same database at the same time when using IMAP. Access is limited to individual clients regarding POP3 since all received e-mails are downloaded to the local computer. The following table compares the differences between IMAP and POP3:

Various applications of both protocols result from the aforementioned differences between IMAP and POP3: since POP3 is only limited to a single client and always retrieves all e-mails, applying the transfer protocol only makes sense when you want to obtain your e-mail account from a local computer. After the e-mails have been downloaded, you don’t need any further connection to the mail server and can therefore read and edit your messages without an internet connection.

If you want to access your e-mails with your smartphone, tablet or other clients, IMAP is the better choice. It is definitely an advantage when you are out and about and can only use your mobile data. Since IMAP only opens desired e-mails, you can choose to open ones with a large amount of content at home on your computer. As no local versions of the e-mails are downloaded, you always need an internet connection. It is possible to do more with the IMAP network protocol than just retrieve e-mails; users can also create and manage folder structures, label the editing status of e-mails, and archive dispatched messages. These extra features and the fact that the e-mails are saved on the server until they are deleted means that IMAP is a bigger burden to the mail server compared to POP3.

Whether you choose IMAP or POP3, your decision should be dependent on the planned usage as well as on the available resources.