EHLO

EHLO, short for Extended Hello, is an SMTP command used by mail transfer agents and clients to identify themselves to an SMTP server and to discover the server’s extended capabilities. It supersedes the original HELO command in the realm of Extended SMTP (ESMTP) and enables negotiation of additional features beyond basic mail delivery.

How it works: When a client connects to an SMTP server, it sends EHLO followed by a domain name or identifier (for example, EHLO example.com). The server replies with a sequence of 250-status lines, starting with 250- for all but the last line and 250 for the final line. Each capability the server supports is listed as part of this response, such as STARTTLS, AUTH, 8BITMIME, SIZE, PIPELINING, DSN, SMTPUTF8, and CHUNKING. If the server does not support ESMTP, it may respond with an error to EHLO and the client should fall back to HELO and legacy SMTP behavior.

Common capabilities advertised via EHLO include STARTTLS (to upgrade the connection to TLS), AUTH (supported authentication mechanisms), 8BITMIME (8-bit data), SMTPUTF8 (UTF-8 mail), PIPELINING (command batching), CHUNKING (DSN support for large messages), SIZE (maximum message size), and DSN (delivery status notification).

Fallback and flow: If a server indicates it does not support ESMTP, the client can retry with HELO. After successful TLS negotiation (STARTTLS), clients typically issue EHLO again to re-advertise capabilities in the secured session.

Example:

EHLO example.com

250-example.com at your service

250-STARTTLS

250-AUTH PLAIN LOGIN

250-8BITMIME

250 SIZE 10485760