SMTP server can be tested simply by using telnet client. On Windows 7 above telnet client has to be installed first via control panel (Windows Add/Remove Features)

Following is a sample SMTP commands to send for a standard mail server listening on IP port 25, without authentication

C:telnet  25
HELO mycompany.com
MAIL FROM:
RCPT TO:
DATA
To: Gerry Tan <>
From: My Company Support <>
Subject: Testing mail server via SMTP
Please ignore this email as this is just testing mail server via SMTP

.
quit

The two newlines and dot at the end is important.

SMTP Server With Authentication

To use SMTP username / password authentication, you first need to encrypt it to Base64. It can be done with command line perl:

 perl -MMIME::Base64 -e 'print encode_base64("gerrytan");'
 perl -MMIME::Base64 -e 'print encode_base64("Mypass123");'

Becareful if you username / password contains symbols meaningful to perl! An @ character can be interpreted as perl array. You have to escape it using (I spent an hour figuring out why authentication failed due to this).

And issue AUTH LOGIN command after HELO / EHLO. The server will prompt for username and password in Base 64

220 mail.tpg.com.au ESMTP (mail16) Sendmail ready.
EHLO mail.tpg.com.au
250-mail16.tpgi.com.au Hello  [], pleased to
meet you
250-ENHANCEDSTATUSCODES
250-PIPELINING
250-8BITMIME
250-SIZE 28521268
250-DSN
250-AUTH LOGIN PLAIN
250-STARTTLS
250-DELIVERBY
250 HELP
AUTH LOGIN
334 VXNlcm5hbWU6
**********
334 UGFzc3dvcmQ6
**********
235 2.0.0 OK Authenticated
MAIL FROM:
250 2.1.0 ... Sender ok
RCPT TO:
RC250 2.1.5 ... Recipient ok

Thanks to http://exchange.mvps.org/smtp_frames.htm.