ESMTP

ESMTP stands for Extended Simple Mail Transfer Protocol. It is an extension to the original SMTP protocol used for sending electronic mail. ESMTP was developed to overcome some of the limitations of basic SMTP and to support newer features and capabilities for email transmission.

The primary purpose of ESMTP is to allow mail servers to negotiate capabilities during the initial connection. This negotiation happens through the EHLO command, which replaces the older HELO command used in basic SMTP. EHLO allows the client to advertise its supported extensions, and the server responds by listing its own supported extensions. This handshake mechanism enables a more robust and flexible email transfer process.

Key extensions supported by ESMTP include 8BITMIME, which allows for the transmission of binary data in email, and SIZE, which informs the sender about the maximum message size the server can handle. Other extensions provide support for authentication (like AUTH), help (HELP), and verification of recipient addresses (VRFY). These extensions enhance security, efficiency, and the overall reliability of email delivery.

ESMTP is now the de facto standard for email transmission between mail servers. Most modern email clients and servers implement ESMTP to ensure compatibility and to leverage the advanced features it offers. The extended capabilities of ESMTP have been crucial in the evolution of email systems, enabling richer content and more secure communication.