/ˌiː-ˈsɛm-ti-pi/
n. “Email with a few extra powers.”
ESMTP, short for Extended Simple Mail Transfer Protocol, is an enhancement of the original SMTP protocol used to send email over the Internet. While SMTP provides the basic rules for transferring messages from one server to another, ESMTP adds a suite of optional extensions that improve functionality, reliability, and security.
Introduced in the early 1990s through RFC 1869, ESMTP allows servers to negotiate additional capabilities during the connection handshake. These extensions include support for authentication (so users can securely send email through a server), larger message sizes, delivery status notifications, and even encryption commands.
For example, a server implementing ESMTP can advertise that it supports STARTTLS for encrypted email transport. When a client connects, it can upgrade the connection from plain text to TLS, preventing eavesdropping. Other extensions allow specifying the maximum message size or requesting delivery receipts, enhancing the control and reliability of email delivery.
ESMTP is backward compatible with traditional SMTP. If a connecting client doesn’t understand the extensions, the server simply communicates using basic SMTP commands. This ensures wide interoperability while allowing modern features when both sides support them.
Many modern mail servers, including Microsoft Exchange, Postfix, and Sendmail, implement ESMTP by default. It’s also used by cloud email providers to support authentication, anti-spam measures, and secure transport mechanisms.
In practice, ESMTP helps prevent abuse and ensures messages are delivered efficiently. By supporting authentication, it prevents unauthorized users from sending email through servers (reducing spam). Extensions like size limits prevent oversized messages from overwhelming servers, and encryption capabilities protect sensitive content during transit.
In summary, ESMTP is SMTP evolved — it keeps the simplicity of the original protocol while adding a toolbox of optional features that make email faster, safer, and more functional. Without it, modern email as we know it — secure, authenticated, and feature-rich — would be far less reliable.