👉 Overview
👀 What ?
SMTP (Simple Mail Transfer Protocol) is a protocol that enables the sending of emails over the Internet. The SMTP commands are the set of instructions used to send, receive, and relay outgoing mail between email senders and receivers.
🧐 Why ?
Understanding SMTP commands is crucial as it helps in troubleshooting email sending and receiving problems. It's also important for email servers administration and for cybersecurity professionals to detect and prevent email-based threats.
⛏️ How ?
SMTP commands are used in the communication between the mail client and the mail server. These commands include HELO (or EHLO) to identify the client, MAIL FROM to specify the sender, RCPT TO to specify the recipient, DATA to transmit the body of the message, and QUIT to end the session. Each of these commands is sent by the client and a response is expected from the server.
⏳ When ?
SMTP was first introduced in 1982 as part of the Internet standards. SMTP commands have been used since then to facilitate email communication over the Internet.
⚙️ Technical Explanations
SMTP works on a request-response model. The client (often an email client) sends a command and waits for a response from the server. The server responds with a three-digit status code followed by a text message. The status code indicates whether the command was successful or not. For example, '250' indicates a successful operation while '500' indicates a syntax error in the command. Each SMTP session starts with the client sending a HELO or EHLO command followed by the domain name or IP address of the client. If the server responds with a '250' status code, the client can proceed with the next command. The MAIL FROM and RCPT TO commands specify the sender and recipient email addresses respectively. The DATA command initiates the transmission of the email body. The QUIT command ends the session. It's important to note that SMTP is not secure by default and can be vulnerable to interception and eavesdropping. Therefore, it's often used with security extensions like STARTTLS or SMTPS.