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ESMTP (Extended Simple Mail Transfer Protocol)

What is ESMTP (Extended Simple Mail Transfer Protocol)?

Extended SMTP, or ESMTP, is an extension to the Simple Mail Transfer Protocol (SMTP). It has the same job as SMTP but adds more functionality and flexibility for email transmission between servers. While SMTP handles basic email delivery, ESMTP allows mail servers to negotiate encryption, authenticate senders, handle large file sizes, and speed up delivery process.

ESMTP provides the advanced features that are needed for modern communication, such as multimedia support, security, and bandwidth efficiency. One of the main usage of Extended SMTP is sending marketing newsletters email that include high-resolution images and a PDF attachment.

ESMTP vs. SMTP: What's the difference?

The original SMTP was designed in 1982. It was simple and reliable, but it wasn't built for modern email features such as encryption, large attachments, or confirming a sender's identity.

ESMTP was designed to address these issues and add new features to email without disrupting how older systems already work.

Here's how they compare:

Key features of ESMTP

ESMTP introduces several important capabilities that are essential for modern email delivery:

Authentication (SMTP AUTH)

Verifies the sender’s identity before sending emails and helps prevent unauthorized usage and spam.

Encryption (STARTTLS)

Upgrades a plain email connection to an encrypted one mid-conversation. Without it, emails are sent as plain text, readable by anyone in between.

Message size declaration (SIZE)

Allows servers to declare the size of email to handle large email sizes more efficiently.

8-bit MIME support (8BITMIME)

Allows emails to include special characters, emojis, and languages that use non-Latin alphabets, along with regular plain text.

Delivery status notifications (DSN)

Gives senders updates on whether an email was delivered, delayed, or failed.

Command pipe lining (PIPELINING)

Allows the server to send multiple commands together without waiting for individual responses.

How does ESMTP work?

Every time an email is sent, the two sending and receiving mail servers have a quick conversation to get the message across.

Here's what that looks like step by step:

1. Greeting (EHLO): The sending server says "hello" to the receiving server using the EHLO command and asks what features it supports.
2. Feature check: The receiving server shares its list of supported features. The sending server chooses what to use.
3. Login (AUTH): The AUTH command is used to send login credentials if needed. The server verifies them before sending email.
4. Encryption (STARTTLS): If both servers support it, the connection is upgraded to encrypted so no one else can read what's being sent.
5. Addressing (MAIL FROM): The sending server specifies who the email is from and who it's going to.
6. Send message (DATA): The actual email with its subject, body, and attachments is transmitted.
7. Goodbye (QUIT): The session ends and the connection closes.

How does ESMTP improve email sending?

Extended SMTP is the standard used by most email systems today. It solves many limitations of basic SMTP, such as lack of security, limited functionality, and lesser support for modern email formats.

Encryption keeps your emails secure

ESMTP supports encryption through STARTTLS to protect emails during transmission.This ensures that sensitive information such as OTPs, passwords or account details can't be read by unauthorized units in transit.

Authentication stops your domain from being spoofed

ESMTP enables authentication using the AUTH command to verify the sender before sending emails. When combined with SPF and DKIM, it tells receiving servers that your emails are genuine and prevents attackers from spoofing your domain.

Extensions help in faster email delivery

ESMTP improves email sending speed by using extensions such as PIPELINING. It allows the server to send multiple commands at once, without waiting for individual responses. This helps when you're sending hundreds of transactional emails a day.

Better bounce handling

With Delivery Status Notification (DSN ), ESMTP gives you timely and actionable reports on email delivery. This helps you work on failed deliveries, removes bad addresses, and protects your sender reputation.

Broad compatibility

ESMTP works with all mail servers. If a receiving server doesn't support ESMTP, it automatically switches to basic SMTP so your email still gets delivered.