telnet test.server 25
If the connection is successful, we should see a banner and a command line interface. You may consult RFC 821 for SMTP commands,
220 test.server Microsoft ESMTP MAIL Service, Version: 5.0.2195.5329 ready at Mon, 15 Sep 2009 20:50:07 +0210
250 test.server Hello [192.168.0.101]
mail from: email@example.com
250 2.1.0 firstname.lastname@example.org....Sender OK
rcpt to: Sam@acc.net
250 2.1.5 Samp@acc.net
354 Start mail input; end with .
subject: test mail
I'm sending email using TELNET.
250 2.6.0 Queued mail for delivery
221 2.0.0 test.server Service closing transmission channel
Connection to host lost.
Press any key to continue...
The above techniques can also be used for NULL sender identity check and SMTP Open Relay.