Understanding How an Email System Works

Emails have become an indispensable medium of communication in this highly connected digital world. Email messages have almost completely replaced the age-old system of sending physical letters by post. Emails have helped us save so much time and increase productivity at work. Sending messages from one end of the globe to another has become a matter of minutes, thanks to this technology.

But have you ever wondered how email works? What happens after you type an email and hit send? Email processing is just as simple as it is complex and is still a mystery for many of us. What goes on behind the scenes of this revolutionary technology is something worth knowing. Not only would it enrich your knowledge but you would also be able to understand why your emails are sometimes delayed or bounced back to you.

What is an email?

Email is a shortened version of electronic mail. It is a method of exchanging text messages and files through electronic devices. The limited use of emails started as early as the 1960s. At that time users could only send messages to other users of the same computer or computers on the same server. In some email systems, it was necessary for both users to online at the same time for the emails to be delivered.

The email system as we know it currently was developed in the 1970s. Ray Tomlinson developed the first system capable of sending messages between users on different hosts over the ARPANET, with the @ sign linking user names with a destination server, in 1971.

The @ symbol separates the user name from the server and indicates that the email is not being sent to a local host.

What is an email server?

Similar to the post office in the case of traditional mailing systems, emails use email servers to route the messages you send. Every email passes through a series of email servers before reaching the recipient. However, unlike traditional mail, there is not just one service for all your mails, and incoming and outgoing messages are handled differently.  So, email servers can be broadly classified into two types, that is outgoing mail servers and incoming mail servers.

The outgoing servers are known as Simple Mail Transfer Protocol or SMTP. Whereas the incoming server can be of two types, namely POP3 or IMAP.


SMTP is an email delivery protocol that is used to send emails over the internet. It uses a process known as “store and forward” which helps the email message to move in several steps from your computer to the destination email server. SMTP works in conjunction with the Mail Transfer Agent or MTA. The SMTP, which is a set of commands that authenticate and direct the email message, determines how your email will move from your MTA to the recipient’s MTA. MTA can also be of two types. A client-based MTA requires you to install software to access your emails, as in the case of Microsoft Outlook. A web-based MTA can be accessed through a web browser, as in the case of Gmail or Yahoo Mail, for example.


POP3 stands for Post Office Protocol, version 3. The POP3 servers store incoming emails on a remote server until the user opens the email client software to check their mail. The messages are then transferred to the user’s email program and deleted from the remote server to make space for more incoming emails. POP3 makes it possible for you to access your emails from anywhere, as long your computer is configured to work with the protocol.


IMAP is the acronym for Internet Message Access Protocol. The IMAP is another protocol that is used to access emails but it is more capable than POP. Unlike the POP server, IMAP servers keep the emails stored on the server itself so that you can access the emails from any device and any place. When you log into your inbox, the email client requests the IMAP server to connect you to your emails. The emails are not downloaded to the user’s system but accessed from the server itself.

How are emails sent and received?

So, once an email is written and the send button is clicked, all the details of the email are sent to your SMTP server by the email client. The SMTP server identifies the recipient’s domain name and if it is a different domain from yours, then the SMTP communicates with the DNS.

 The next stage is the DNS lookup. DNS stands for Domain Name System and its function is to translate the domain names, such as “” or “”, to IP addresses or vice versa. The IP address is essential when sending emails across different domains. The IP address is a specific, unique number assigned to every computer connected to the internet. Your SMTP cannot send a mail accurately without knowing the IP address of the recipient on another domain.

 When the DNS lookup is completed and the recipient’s IP address is found, the DNS sends a response to the sender’s SMTP server with the recipient’s IP address. Now since your SMTP server has the destination IP address, it can connect to the recipient’s SMTP server. The message is then relayed through a number of different servers until it reaches the recipient’s SMTP server.

The destination SMTP server will read the incoming message and identify the domain name and user name of the recipient. It then delivers the email to the recipient domain’s POP3 or IMAP server. The message is queued in this server until the recipient opens the email through the client’s software.

If the sender and the recipient are on the same domain, however, the process becomes much simpler. In such a case, the email can directly be sent to the domain’s POP3 or IMAP server and there is no requirement of routing between servers or even of a DNS lookup.

If there is a large volume of incoming emails in the recipient’s email server or if the incoming emails are unusually large, your email delivery may get delayed. The recipient’s MTA will first process the emails that are ahead of yours in the queue. If there are any errors in the email, particularly in the recipient’s address, the servers will repeatedly try to send the mail forward. When the sending permanently fails, the server sends a bounce back mail to your inbox with the reason for failure.


The process of sending an email may seem like a trivial day-to-day business for us. But the amount of thought that has gone into building this system is commendable. It is good to have a basic know-how of the technology and the steps involved in email processing.

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