What is SMTP?

About

The SMTP protocol works via a series of SMTP command between the server and the client.

Each command has:

The server will then respond with a reply code and a human text that defines if the command was successful.

For an example on how to transmit a message with the SMTP protocol, see this howto:

for reception: ie to the final destination SMTP server known as the MDA (Mail Delivery Agent)
for sending/relay: ie to the MTA (Mail Transmission Agent) SMTP server that will deliver it to the MDA (Mail Delivery Agent)

HELP - Get the lists of command
HELO, - same as EHLO (deprecated)
EHLO - Extended Helo - identify the SMTP client to the SMTP server with the the fully-qualified domain name as argument

of the SMTP client, if one is available.

ETRN,
MAIL FROM, Identifies the return address (not the sender, the sender is in the message)
RCPT TO, Identifies the recipients
VRFY, verify the username exists on the servers
STARTTLS is a StartTLS m to secure the connection with TLS. It takes no parameters. For PostFix, see How to configure Postfix for TLS / SSL and StartTLS?). With mta-sts, you can define your policy publicly.
DATA (identified start of the message ie followed by a the body email
BDAT: send the data in chunk, not line by line as DATA does.
RSET - Reset, abort transaction and discard info
NOOP - No operations, only elicits an OK from the server used to check the connection.

Other:

Generally forbidden (postconf.5.html)

Login: