What is SMTP?

Have you ever wondered how your email gets to its recipient?

Embedded at the heart of email communications is the Simple Mail Transfer Protocol (SMTP), which has powered the online messaging system for decades.

This protocol is best described as a set of principles that outline how data can be transmitted between email servers in order for messages to be sent from account to account. The most widely-used email systems, including the likes of Outlook.com and Gmail, use SMTP to power communication so emails can be sent and received from beyond the confines of their own servers.

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SMTP, also known as RFC 821 and RFC 2821, was first devised by the Internet Engineering Task Force (IETF) in the 80s by a wide-spanning community of vendors, operators, researchers, engineers and network designers. These standards were initially part of the application layer of the TCP/IP protocol.

The four components of SMTP include a local user or client-end utility known as the mail user agent (MUA), a server called the mail submission agent (MSA), the mail transfer agent (MTA), and a mail delivery agent.

The protocol shifts an email from one device’s MTA to another’s by using a ‘store and forward’ mechanism. The message travels through the network using a chain of these store and forward actions before getting to where it needs to.

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Depending on user settings, the Post Office Protocol 3 (POP3) or Internet Message Access Protocol (IMAP) mechanisms are deployed to gather and transfer emails to the intended.

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There are benefits and drawbacks to both, although enterprise applications normally make use of IMAP - if required - because emails can be stored on a server instead of individual devices. It’s important to specify which server messages are stored on so they can be received properly by each device.

The SMTP server

An SMTP server is just a computer running SMTP. Once messages are picked up they are sent to this server for passing onto recipients.

Six steps to (email) heaven

An email is sent from sender to recipient using six steps.

1) The email is submitted by an MUA to a mail server's MSA.

2) This email is transferred to the server's MTA via port 25 (the MTA and MSA are typically hosted on the same SMTP server).

3) The MTA confirms the MX record of the recipient domain and transfers the message to another MTA (this can happen many times until the message is received by the destination server).

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4) The email is given over to the MDA, which saves the message in the correct format for retrieval by the receiving MUA.

5) The receiving MUA requests the message from the MDA via POP3 or IMAP.

6) Lastly, the email is delivered to the receiving MUAs inbox.

Using MIME to send attachments

SMTP is a very reliable protocol, but it is constrained to sending text only. This would make email very limited as we wouldn't be able to send any attachments. This is where MIME comes in.

MIME stands for Multipurpose Internet Mail Extensions. This is an extension to SMTP that allows users to send different kinds of files over the internet, such as audio, video, images, application programs, etc. It also allows for non-Latin characters and emails that contain multiple parts (such as an email with a plain text element and an HTML element).

SMTP Authentication

Another extension to SMTP is SMTP Authentication. This enables email clients to authenticate when connecting to an SMTP server using a username and a password. This prevents unauthorised users from sending email through an SMTP server.

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