Why is SMTP server used?

Is SMTP protocol still used?

The SMTP protocol is used on the internet, but not on the LAN.

SMTP servers typically sit on the internet, but when you send email, it's sent through a local SMTP server on your LAN. The LAN is generally a private network, and is where all your servers (like email servers, web servers, etc.) sit. When you send email from one server to another server, you are using a private connection over the LAN, not the internet.

This does not mean that SMTP is "dead", because there are still many internal email systems in place. The email system for a company can be very complex, and it's not uncommon for the same company to have multiple systems, each of which handles email differently. That's why you see some of the systems handling email differently.

Why is SMTP server used?

SMTP protocol is used by many mail programs, and not only to send email from a specific server, but also to relay email messages through other servers in a chain.

Most major Email clients (eg Microsoft Outlook, Apple Mail, Yahoo! Mail) use this protocol, as it is the best way to send email from one computer to another on the internet. SMTP is also the protocol used by most Email services, for example Gmail, Hotmail and AOL provide webmail with built-in SMTP server, for you to easily send an email without needing to set up a new account.

How does SMTP work? Since SMTP protocol is built on top of TCP (Transmission Control Protocol) over IP, it operates with the following rules: The sender initiates the communication by establishing a TCP connection to the recipient. TCP connection request is authenticated. A connection-less delivery protocol is used. Data is sent between connected hosts in a stream. To send email, the message needs to be converted to the SMTP format, which consists of several lines, formatted with CRLF. Each line has 5 pieces of information: The name of the host to be sent to, or received from. The message number or identifier. The line of the mail sender. The recipient's address. The line of the recipient. These lines are put into a single multi-line text, which will be formatted for transmission over a network. If a mail client receives the data, then the program can process it to decode the lines of information and present them in the form of plain text, in a window.

The TCP layer on a network has no concept of packets or streams, thus, instead of sending the whole email body in one continuous stream, SMTP provides a packet-like service. The packet is defined by a five-byte header, containing the information needed to deliver the packet to its destination, as well as the actual data.

SMTP Header. Header is divided into two parts: Status. Data field is defined as a sequence of eight two-byte values (one byte containing the length of one octet, another containing a parity bit).