SMTP is a traditional command based Internet protocol; the client issues command verbs with one or more arguments, and the server responds with a 3 digit status code and additional information. The response can span one or multiple lines, the continuation is indicated by an '-' character between the status code and text.

The communication itself is stateful, the client first specifies the sender via the "MAIL" command, then the recipients using multiple "RCPT" commands. Finally it sends the mail body using the "DATA" command. After a transaction finishes the client either closes the connection using the "QUIT" command, or starts a new transaction with another "MAIL" command.

220 mail.example.com ESMTP Postfix (Debian/GNU)
EHLO client.host.name
250-mail.example.com
250-PIPELINING
250-SIZE 50000000
250-VRFY
250-ETRN
250-XVERP
250 8BITMIME
MAIL From: <sender@sender.com>
250 Sender ok
RCPT To: <account@recipient.com>
250 Recipient ok
RCPT To: <account2@recipient.com>
250 Recipient ok
DATA
354 Send mail body
From: sender@sender.com
To: account@receiver.com
Subject: sample mail

This is a sample mail message. Lines beginning with
..are escaped, another '.' character is perpended which
is removed when the mail is stored by the client.
.
250 Ok: queued as BF47618170
QUIT
221 Farewell