How can I test my SMTP service ?

You can use TELNET to test your SMTP service. SMTP uses port number 25, so in order to use telnet to open an SMTP session with an Exchange server we use the command

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
helo test.server
250 test.server Hello [192.168.0.101]
mail from: admin@test.com
250 2.1.0 admin@test.com....Sender OK
rcpt to: Sam@acc.net
250 2.1.5 Samp@acc.net
data
354 Start mail input; end with .
subject: test mail
Hi Sam,
I'm sending email using TELNET.
.
250 2.6.0 Queued mail for delivery
quit
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.