Messaging protocols: SMTP, POP, IMAP

Understand the difference between SMTP, POP3 and IMAP messaging protocols

The three main protocols used by a mail server are:-
SMTP (Simple Mail Transfer Protocol),
POP (Post Office Protocol) and
IMAP (Internet Message Access Protocol).
There is yet another, the MAPI (Messaging Application Programming Interface) but which is only used when using Microsoft Exchange.

The SMTP messaging protocol

This communication protocol is used for the transfer of electronic messages on the network. It is a client/server type. Each request for sending by the client is followed by a response from the server. It is a simple protocol that uses the TCP transmission control protocol for data transfer. The exchanges of mails on a mail server are done via ports (a door for the server) and the SMTP protocol listens, by default, to port 25 with the aim of routing the messages. Read below to understand precisely how the SMTP messaging protocol works.
The Simple Mail Transfer Protocol, better known under the name of SMTP, is an e-mail protocol which aims to forward emails to the e-mail servers so that users can consult their e-mails.

How SMTP works

Sending and receiving e-mails is done via ports, by default, port 25 is used by e-mail servers and port 587 by e-mail clients. Different commands make up the scenario for sending emails so that they can pass directly from the sender to the recipient.

Here, everything is a question of syntax, the slightest error in the code could generate a blocking of emails. If the SMTP messaging protocol is the most used, it still encounters certain limits. In fact, it does not identify the sender of emails and does not help fight spam, a scourge on the web for several years.

Transfer agents (MTA)

Mail transfer agents (MTAs) or mail servers are the programs that allow mail to be transferred between servers. An email can very well be relayed between several MTAs. It is possible to know all the MTAs through which the mail has passed by displaying the source of the message. Here are some of the most common:


It was Sendmail which was the first mail server to use SMTP in 1983. Today, it is the most used MTA in the world with around 55% of SMTP servers. Very reliable, Sendmail is the most powerful MTA but also surely the most criticized for its slowness and its complexity in its implementation and its maintenance.


Postfix is one of the best alternatives to Sendmail. Published under a free license, it remains however incompatible with the GPL license.

Other MTAs

There are other lesser-known servers like Exim (EXperimental Internet Mailer) and qmail.

POP email protocol

The POP protocol (Post Office Protocol) is now available in version 3, also called POP3. This is the standard protocol which allows the recovery of mails located on a remote server (POP server). The objective of this protocol is to collect e-mail from a host that does not contain its mailbox. It simply downloads messages from the server and stores them on the workstation.

The advantage of this protocol is to allow the consultation of his mail in “offline” mode, without the need for a permanent internet connection. The disadvantage, on the other hand, is that it is not suitable for mobile devices (smartphones, tablets, SaaS) and that the messages are not permanently synchronized with the server.

IMAP messaging protocol

The IMAP protocol (Internet Message Access Protocol) is a bit the opposite of the POP protocol, that is to say, that it has a constant connection to the mail server to be able to consult its mails. This protocol permanently synchronizes the messages contained on the server and on the workstation. Its advantage, therefore, lies in the possibility of consulting emails from anywhere and being able to synchronize and save messages on the server.

Which protocol to choose between POP and IMAP?

The big difference between the POP and IMAP messaging protocols is that POP downloads messages to the server and stores them locally, on your workstation, while the IMAP protocol operates a constant synchronization between your workstation and the server.

The IMAP protocol is often more relevant for several reasons:

The messages remain stored on the server, so they are saved within the limits of the existing storage on your server. In the event of a problem on your workstation, none of your emails will be lost.

IMAP manages remote subfolders, that is, you can find your mailbox organization everywhere.

Permanent synchronization allows very fine management of your messages. For example, if you send a message in “unread” on your mobile, it will also be automatically on your workstation.

And security in all of this?

The two protocols POP and IMAP exist in secure version, POPs (POP3 over SSL) and IMAPs (IMAP over SSL). This type of security is recommended especially in the context of WiFi use, including at home, to avoid malicious recovery of your identification data (login and password).

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