MailEnable Standard Guide
How Internet Email Works
Introduction > How Internet Email Works

To administer a mail server on the Internet requires knowledge of how email works. It is important to know how messages are delivered and sent, how mail servers contact each other, and how users retrieve their email. This will help in diagnosing problems, tracking faults, and knowing who to contact when something goes wrong. The information in this section is not specific to MailEnable; this applies to all mail servers. This information is essential to know in order to properly administer an Internet mail server.

Email Clients

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

Some examples of email clients include

Email server

An email server holds and distributes e-mail messages for email clients. The email client connects to the email server and retrieves messages.  An email server may also be known as a mail server, or a mail exchange server.

Sending and receiving mail

To send Internet e-mail, requires an Internet connection and access to a mail server. The standard protocol 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 SMTP server.  If the recipient of the email is local (i.e. at the same domain as the email originated from) the message is kept on the server for accessing by the POP, IMAP or other mail services for later retrieval.

If the recipient is remote (i.e. at another domain), the SMTP server communicates with a Domain Name Server (DNS) 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 SMTP server sending the mail is unable to connect with the remote SMTP 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 (30 hours by default), the message will be returned to the sender as undelivered.