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Do you know how email works?

Posted On: Nov 25, 2014

 Knowledge on how email works?


If you are interested to know how messages are delivered and sent, how mail servers contact each other, and how users retrieve their email. This will help you, and knowing who to contact (or blame!) when something goes wrong. The information in this section is not specific to one or other mail server or service providers. This is written purely educational purpose. In the process of writing an email, sending and receiving, viewing an email there are mainly two components involve and they are “Email Client” and “Email Server”  


What is an Email Client?  

An email client is a software application that is used to send, receive, store and view an e-mail.

Some examples of email clients are:

  • Microsoft Outlook
  • Microsoft Outlook Express
  • Thunderbird
  • AOL Mail
  • Lotus Notes

Sometimes you may also use your browser as a mail client. For example you can access your Gmail, Hotmail and Yahoo accounts using your web browser such as Internet Explorer, FireFox or Google Chrome

What is an Email Server?

An email server holds and distributes e-mail messages for email clients. A mail / email server (AKA a mail transfer agentor MTA, a mail transport agent, a mail router or an Internet mailer or mail exchange server) is an application that receives incoming e-mail from people within the same domain & remote senders and forwards outgoing e-mail for delivery to respective recipient. A computer dedicated to running such applications is also called a mail server.

The email client connects to the email server and retrieves messages. There are many email servers are available in the market that you can buy. Microsoft Exchange Server, q-mail, Exim, MailEnable, MDeamon and Sendmail are some of the many mail server programs.

Transporting the Email (Sending & Receiving)

To send Internet e-mail, requires an Internet connection and access to a mail server. The standard protocol (set of rules or a program on your computer) used for sending Internet e-mail is called SMTP (Simple Mail Transfer Protocol).  The SMTP protocol is used to both send and receive email messages over the Internet.

  When a message is sent, the email client sends the message to the mail server using Simple Mail Transfer Protocol.  If the recipient of the email is local (i.e. in same domain as the email originated from) the message is kept on the server for accessing by the POP (stands for Post Office Protocol), IMAP (stands for Internet Mail Access Protocol) or other mail services for later retrieval.  This is a mail protocol that enables emails to be retrieved from a remote mailbox. Often both SMTP and POP services would be running from the same server (or computer). However, in some cases, one server is used for receiving mail (AKA POP server) and another server is used for sending mail (AKA SMTP server).

If the recipient is remote (i.e. at another domain), the mail server communicates with a Domain Name Server (DNS) (of the remote domain) to find the corresponding IP address for the domain being sent to.  Once the IP address has been resolved, the SMTP server connects with the remote SMTP server and the mail is delivered to this server for handling.


If the mail server sending the mail is unable to connect with the remote mail server, then the message goes into a queue.  Messages in this queue will be retried periodically.  If the message is still undelivered after a certain amount of time (let us say 30 hours by default), the message will be returned to the sender as undelivered.