IMAP or POP3 – comparing e-mail protocols

E-mails are trans­port­ed via mail servers and are received with the help of e-mail programs (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.