One of the primary services offered by an ISP is email hosting. Email has revolutionized how people communicate through its simplicity and speed. Yet to run on a computer or other end device, email requires several applications and services.
Email is a store-and-forward method of sending, storing, and retrieving electronic messages across a network. Email messages are stored in databases on mail servers. ISPs often maintain mail servers that support many different customer accounts.
Email clients communicate with mail servers to send and receive email. Mail servers communicate with other mail servers to transport messages from one domain to another. An email client does not communicate directly with another email client when sending email. Instead, both clients rely on the mail server to transport messages. This is true even when both users are in the same domain.
Email clients send messages to the email server configured in the application settings. When the server receives the message, it checks to see if the recipient domain is located on its local database. If it is not, it sends a DNS request to determine the IP address of the mail server for the destination domain. The email is then forwarded to the appropriate server.
Email supports three separate protocols for operation: Simple Mail Transfer Protocol (SMTP), Post Office Protocol (POP), and Internet Message Access Protocol (IMAP). The application layer process that sends mail, uses SMTP. This is the case if sending from a client to a server, as well as when sending from one server to another.
A client retrieves email, however, using one of two application layer protocols: POP or IMAP.