telnet mx1.example.com smtp
// Trying 192.0.2.2…
// Connected to mx1.example.com.
// Escape character is ‘^]’.
// server: 220 mx1.example.com ESMTP server ready Tue, 20 Jan 2004 22:33:36 +0200

HELO client.example.com
// server: 250 mx1.example.com

MAIL from: <sender@example.com>
// server: 250 Sender <sender@example.com> Ok

RCPT to: <recipient@example.com>
// server: 250 Recipient <recipient@example.com> Ok

DATA
// server: 354 Ok Send data ending with <CRLF>.<CRLF>

From: sender@example.com
To: recipient@example.com
Subject: Test message
This is a test message.
.
// server: 250 Message received: 20040120203404.CCCC18555.mx1.example.com@client.example.com

QUIT
// server: 221 mx1.example.com ESMTP server closing connection