What is the purpose of the EHLO command in Extended SMTP (ESMTP), which…
2025
What is the purpose of the EHLO command in Extended SMTP (ESMTP), which enhances the basic SMTP protocol?
- A.
It replaces the HELO command and announces support for extended SMTP features.
- B.
It encrypts the email content before transmission.
- C.
It initiates the direct transfer of binary files without MIME encoding.
- D.
It checks the availability of DNS servers before sending email.
Attempted by 153 students.
Show answer & explanation
Correct answer: A
The original Simple Mail Transfer Protocol (SMTP, RFC 821) used the HELO command for a client to introduce itself to a server. However, basic SMTP lacked support for modern necessities like authentication, file-size limits, and encryption.
To solve this without breaking backward compatibility, Extended SMTP (ESMTP) was introduced.
Note for More Understanding
The Handshake: When an ESMTP-capable client connects to a mail server, it sends EHLO instead of HELO.
The Server's Response: If the server understands ESMTP, it responds with a
250 OKstatus code followed by a list of keyword extensions it supports. These extensions commonly include:SIZE: Declares the maximum message size the server can accept.AUTH: Enables secure mechanisms for user authentication.STARTTLS: Indicates that the server can establish a secure, TLS-encrypted communication channel.8BITMIME: Allows the transmission of 8-bit data characters.
If the server is old and does not recognize EHLO, it returns an error. The client then automatically falls back to sending the classic HELO command, ensuring mail still delivers using the basic protocol rules.