Programming Internet Email

ESMTP Commands

ESMTP consists of a minimal core of required commands (listed in Table 9-1) and a number of commands associated with the service extensions. Some of the service extensions also extend existing commands but do so without breaking backward compatibility. As with the rest of the protocols and formats in the Internet mail system, backward compatibility has been a key design criterion.

Table 9-1. ESMTP Commands That May Occur at Any Time

Command	Arguments	Meaning	Status
HELO	Domain name	Used to identify the client.	Required
EHLO	Domain name	ESMTP replacement for HELO. Used to identify the client and request a list of ESMTP service extensions supported by the server.	Optional
VRFY	A username or email address	Asks the server if it can verify that the username or email address given maps to a local user.	Optional
EXPN	Email address	Asks the server if it can expand the email address given if the address is a mailing list.	Optional
HELP	Optional command name	Asks for a general help message or help on the command given.	Optional
NOOP	None	No operation (does nothing).	Required
TURN	None	Requests that the client and server exchange roles.	Optional
VERB	None	Requests that the server use verbose mode.	Proposed
ETRN	Hostname, domain name, or queue name	Processes the mail queue for the specified host, domain, or queue.	Proposed
QUIT	None	End the session.	Required

All ESMTP commands are case insensitive.

A minimal SMTP server must support the following commands:

HELO

Announces a client to a server

MAIL

Informs a server of a sender’s ...