How to test an SMTP server using telnet

ref: http://www.anta.net/misc/telnet-troubleshooting/smtp.shtml

telnet: > telnet mx1.example.com smtp
telnet: Trying 192.0.2.2...
telnet: Connected to mx1.example.com.
telnet: Escape character is '^]'.
server: 220 mx1.example.com ESMTP server ready Tue, 20 Jan 2004 22:33:36 +0200
client: HELO client.example.com
server: 250 mx1.example.com
client: MAIL from: 
server: 250 Sender  Ok
client: RCPT to: 
server: 250 Recipient  Ok
client: DATA
server: 354 Ok Send data ending with .
client: From: sender@example.com
client: To: recipient@example.com
client: Subject: Test message
client:
client: This is a test message.
client: .
server: 250 Message received: 20040120203404.CCCC18555.mx1.example.com@client.example.com
client: QUIT
server: 221 mx1.example.com ESMTP server closing connection