How Email Server Works

E-Mails have become an integral part of our daily routine. We can communicate with anyone in world who has an Internet connection and an e-mail account. E-mailing is so easy, thats why people think that they don’t need to understand about the email’s working. But i think you should know about this process to better understand how your mails are going from your system to others system and from others system to your system. So here i am going to show you how your mail server works.

Mail server is a computer which is used to interchange the emails between the systems. There are some parts of emails that are necessary for emails to work. Before knowing how all these parts work,lets have a look what all they work.

1] MUA: It stands for Mail User Agent. It is used to read and write emails. It is also known as email client. Novell Evolution, Mozilla Thunderbird,Microsoft Outlook are example of MUA.

2] MTA: It stands for Mail Transfer Agent. It is used to transfer email between server and clients.It is also called as outgoing mail server. Sendmail, Postfix, Qmail are example of MTA.

3] MDA: It stand for Mail Delivery Agent. It retrieve message from MTA and drop the message into users mailbox. It is also called as incoming mail server. Procmail is the example of MDA, which is installed and running by default in most of systems.

There some protocols that are required for above parts to work, and they are:

1] Smtp: It stand for Simple Mail Transfer Protocol which is used to send the emails. Smtp protocol uses port no. 25.

2] Pop3: It stands for Post Office Protocol (version 3) which is used to retrieve emails.Pop3 uses port number 110 and pop3s ( secure communication using SSL{Secure Socket Layer} ) uses port number 995.

3] Imap: It stands for Internet Mail Access Protocol which is also used to retrieve emails. Imap uses port number 143 and Imaps ( Secure communication using TLS{Transport Layer Security}) uses port number 993.

But there is a difference in above mail retrieving protocols, Pop3 and Imap.


1] All the emails which you access is stored on your system by your email client like Evolution, Mozilla Thunderbird, Microsoft Outlook etc.

2] You can check your previously accessed email when you are offline because the emails are actually stored on your own system, thats why you do not need to be connected to Internet in order to check your previously accessed emails.

3] You can not check your previously accessed email from other locations because the emails are actually downloaded to your system and have removed from the email server.

4] Checking email with the POP3 is little bit slower as compared to IMAP, because POP3 downloads the emails which requires time.

5] It’s secure version is POP3S


1] All the email you access is stored on your email server and never downloaded to your system.

2] You cannot check your previously accessed email when you are offline, because the emails are actually stored on email server.

3] You can check your previously accessed emails from any location of world, the only thing you need is connectivity with Internet.

4] Checking emails with IMAP is much more faster than POP3, because it doesn’t downloads the emails on system.

5] Its secured version is IMAPS.

Now lets see how all this parts and protocols work together is order to send and receive emails.

Email is written and send using MUA like Microsoft outlook, Novell evolution, Mozilla thunderbird. When the MUA writes the email, it forward the email to MTA like Sendmail & Postfix to transfer it to the target user’s Inbox. The MTA transfers the email with the help of SMTP protocol using port number 25.
Now suppose my email address is “” and I am sending email to the address “”. Now when the email is handed over to SMTP, it divides this email address in two parts:

1] Username: Here it is xyz
2]Domain name: Here it is

Now SMTP look whether this domain is parts of his own domain or not. If it founds that this domain is its own domain, then it searches for the username which is registered with its domain. Now finally when the inquiry successfully completes, MTA forwards the message to the MDA. MDA stores the emails of user and protect it using the username and password of user. Now the email retrieval protocols come into the play. Pop3 or IMAP whichever protocol is used by the users MUA, accepts the message from the MDA and shows it to the client.

But what if the both domain are different? Not a big deal !! Keep reading next lines to clear the doubt.

Now suppose my email is “” and I am sending email to “”. Now again MTA send the email using SMTP. SMTP look at the email address. In this case, this domain is different from domain “”, so this time SMTP of “” contacts the DNS(Domain name server). DNS tells the IP address of domain “”. With the help of this IP address, MTA of “” contacts with the MTA of “” and hands over the email to that. Now the MTA of “” passes the email to the MDA, and finally the target user sees its email using MUA, which is also known as Email client.

Note: When you send or recieve email using browser, you are actually using Webmail, not MUA. Gmail, Yahoo, Hotmail, Rediffmail etc..are example of a webmail.

This is how an email works. In our next article we will see what is Sendmail server and how to configure it.

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