Simple Mail Transfer Protocol (SMTP)

SMTP stands for Simple Mail Transfer Protocol. It is a protocol used to send and receive email messages between servers and clients. SMTP is a standard protocol used by email clients and servers to send and receive email messages. The protocol was first introduced in 1982 and has since undergone several revisions.

SMTP is a simple text-based protocol that uses TCP/IP to transfer email messages from one server to another. SMTP clients use port 25 to connect to the SMTP server. The protocol is designed to be reliable, efficient, and extensible.

How SMTP Works

SMTP works on the client-server model. When a user sends an email message, the email client contacts the SMTP server to deliver the message. The SMTP server then contacts the recipient's SMTP server and delivers the message. The recipient's email client then retrieves the message from the SMTP server.

SMTP uses a series of commands and responses to transfer email messages between servers and clients. The protocol includes several command verbs, such as HELO, MAIL, RCPT, DATA, and QUIT. Each command verb is followed by a parameter that provides additional information about the message.

SMTP Extensions

SMTP has undergone several extensions over the years to improve its functionality and security. Some of the notable extensions include:

- SMTP-AUTH: This extension provides authentication of SMTP clients and servers using username and password.
- STARTTLS: This extension provides encryption of SMTP communication between clients and servers.
- ESMTP: This extension adds additional command verbs to SMTP, such as EHLO and SIZE.

Conclusion

SMTP is a critical protocol used in email communication. It provides a reliable and efficient way to send and receive email messages between servers and clients. SMTP has undergone several revisions and extensions to improve its functionality and security.