Email is a means of sending electronic messages between devices. Emails were among the first uses of the Internet, and although they originally were sent only as plain text messages, they now can incorporate graphics, video, or any other type of file. Email addresses are written in a specific way, so that the software sending the email, known as the "client," can determine how to direct them.
Because computers are not running all the time, email has to be held on mail servers until it's requested. Mail servers work the same in principle as physical mail boxes -- they are computers that hold email on their hard drives until they receive a request to pass the mail on to the correct person. Many mail programs, such as Outlook, do not download email to the computer by default; the email is accessed from the mail server every time the program is loaded.
Email addresses are always in the same form -- for example: email@example.com. The address is made up of two parts, the username and the domain name, which are separated by the @ symbol. The domain name explains what the top-level domain is, such as .com or .net, as well as the second-level domain -- in this example, "hotmail." The user name represents some person or group that has an account at this domain and is able to receive email there.
Mail Submission Servers
When an email is sent, a mail submission server looks up the "IP" (Internet Protocol) address associated with the email address. It searches the Internet for "name servers" -- computers that hold information on the IP addresses for a particular top-level domain. In the above example, the client would find a .com name server and look up the IP address for "hotmail.com," then attach that IP address to the email in a manner similar to the way physical mail is labelled with its physical address.
After obtaining the IP address and attaching it to the message, the client relays the email to its destination via several routers, which perform a similar job to sorting offices for physical mail. It is then sent to the destination mail server, which is where the user name section of the email address comes into play. The email is placed in the specific mail box associated with the user name on the email address, ready for the intended recipient to access.