Email can use the application layer protocols POP and SMTP, as shown in the figure. POP and POP3 are inbound mail delivery protocols and are typical client/server protocols. They deliver email from the email server to the client (MUA). The MDA listens for a client to connect to a server. After a connection is established, the server can deliver the email to the client.
The Simple Mail Transfer Protocol (SMTP), on the other hand, governs the transfer of outbound email from the sending client to the email server (MDA), as well as the transport of email between email servers (MTA). SMTP enables email to be transported across data networks between different types of server and client software, and makes email exchange over the Internet possible.
The SMTP message format uses a rigid set of commands and replies. These commands support the procedures used in SMTP, such as session initiation, mail transaction, forwarding mail, verifying mailbox names, expanding mailing lists, and the opening and closing exchanges.