Simple Mail Transfer Protocol: Definition & Uses

Instructor: Lyna Griffin

Lyna has tutored undergraduate Information Management Systems and Database Development. She has a Bachelor's degree in Electrical Engineering and a Masters degree in Information Technology.

In this lesson, we'll learn the basics of Simple Mail Transfer Protocol (SMTP). We will understand its definition, the different components involved and how it is used. We'll also understand how emails are initiated and delivered.

Overview: What is SMTP?

SMTP (Simple mail Transfer Protocol) is a network protocol(a set of rules employed in sending messages over a network) used for the sending and receiving of text-based messages or email messages between one server and another. If you have used an email system before, then SMTP has facilitated the transmission and delivery of your messages. But it must be noted that SMTP only serves to get the messages to their destination recipient servers that is why it is an end to end service. In order for you to download or access these messages, an email client such as Outlook Yahoo or Gmail Protocols is used. An email client is a form of email reader that allows the user to access and manage their messages. The SMTP can be likened to the mailman who gets your mail from the post office to your home/office mailbox. The postman has no capability to receive, read, respond to or throw your mails away. In other words the postman cannot manage your emails. He delivers and that's it. You need an email client to do that.

How Does It all work?

SMTP functions as a delivery system from one SMTP server to the destination SMTP server. This is illustrated in Figure 1. There are different layers in the network protocols employed in internet messaging. SMTP, however, is an application layer protocol (function over the application layer) that manages its messages over the TCP/IP (Transmission Control Protocol/Internet Protocol) Port 25. TCP/IP is a collection of communication protocols that are employed in the interconnection of communication devices over the internet or over a private network.


SMTP Server Contact Initialized

Claire's mail client through her corresponding SMTP server sends a message to Ben's SMTP Server. The server respond acknowledging request with a 220 READY FOR MAIL message Claire's mail client receives the 220 message and responds with a HELLO command. Ben's server responds with '250 requested mail action OK'. The process now begins.

Mail Identification Details Transmitted

With the MAIL command, sender and identification details are sent as a well as details of an address, in case there is an error in transmission. When the Mail command completes successfully the sender transmits a number of RCPT (receipt) commands identifying email recipients. The receiving server responds with 250 OK acknowledging details or in the event there is an error responds with 550 NO such users here.

Mail Details Transmitted

The RCPT commands are completed and the DATA command is executed which informs Ben's SMTP server that the Claire's mail client is ready to transfer the complete email message. The process is started by the 354 START MAIL command. Each line of the message is sent sequentially. An indication of the end of the message is provided by the sender using a sequence of terminal characters: carriage return, line feed, period, carriage return, and line feed.

Message Sent and Process Terminated

On completion, the receiving server will respond with a 250 OK command. In the event that an error occurred, details of the error will be transmitted at this point.

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