DRAFT

You will find here an introduction to the following e-mail protocols: SMTP, POP, IMAP and MAPI. 

The SMTP protocol. The purpose of SMTP (Simple Mail Transfer Protocol) is to route emails sent from a client to the mail server, using TCP connections. The mail server sends it via SMTP to the recipient's mail server. The SMTP server relies on the DNS and consults the MX record associated with your domain: this record indicates the address of the mail server to be contacted. The SMTP protocol uses port 25 by default. In its secure version, known as SMTPS or SMTP over SSL, the port used is 587. You may have heard about SMTP relays. This generally refers to a server through which emails are routed to reach their destination, like an intermediary that facilitates the sending of an email from a piece of equipment or an application.

POP protocol. POP (Post Office Protocol) is now in version 3 and is commonly referred to as POP3. Its purpose is to enable you to download emails to your mailbox from a mail server, to your email client (Outlook, Thunderbird, etc.). This protocol downloads emails on the client and, unless you choose to keep your emails on the server, POP downloads your emails and removes them on the server (that is why it can only be used on a single computer). POP cannot manage a folder structure. This mode of operation means that the POP protocol is not suitable if you use your mailbox on several devices! The advantage is that POP is faster, especially on weak internet connections. Note that with 2SV enabled those who are using a POP account will soon no longer be able to use Outlook to access their Gmail account. Only IMAP accounts are compatible. POP3 will not work in Apple Mail. GTIS recommends avoiding POP3 if at all possible, unless you have a very weak and unstable internet connection. The POP protocol uses TCP connections. It uses port 110 by default, and for the encrypted version, POP over SSL, port 995. This is how to configure Gmail clients with POP3. 

IMAP protocol. Currently in version 4, IMAP (Internet Message Access Protocol) is designed, like POP, to retrieve emails from your mailbox from a mail server. However, it works differently.

This protocol permanently synchronizes e-mails on the client from the mail server. In other words, it creates a copy of the server content on the endpoint. The original message remains on the mail server, making it possible to centralize all mailboxes. It also manages the synchronization of folders you create to organize your mailboxes. Synchronization ensures that your mailbox is in the same state on all your devices. For example, if you set a message on your smartphone to "unread", the same message will also be in the "unread" state on your PC. The disadvantage is that IMAP is too slow on weak internet connections

Based on TCP connections, IMAP uses port 143 or 220, depending on the protocol version.

For the secure version, known as IMAPS, the port is 993.

Please refer to this webpage for more information from GTIS and a comparison between IMAP and POP.

The MAPI protocol (Messaging Application Programming Interface) protocol is a proprietary messaging protocol developed by Microsoft. It is used to carry out communications between an e-mail client and an Exchange mail server. Whether it's an on-premise Exchange server or Exchange Online via Microsoft 365, communications are based on this protocol. The MAPI protocol is used by Outlook. The advantage of MAPI over IMAP is that it incorporates a collaborative aspect: in addition to synchronizing your emails, it also synchronizes your contacts and calendar! The MAPI protocol relies on HTTPS web streams, so it uses the default port for this type of communication, i.e. 443 with TCP connections.