It is quite easy to test an SMTP server using the telnet program. On most computer systems, a connection can be established using the telnet command on the client machine.

Example without authentication

The following command opens a TCP connection from the sending machine to the mail server listening on port 25 on host www.example.com.

telnet www.example.com 25 
The client then waits for the server’s response which will be of the type: 
220 example.com ESMTP 
If the server does not accepts the connection, the client quits 
The client then sends a Mail request showing the envelope sender address, and waits for the  
response: 
MAIL FROM:<postmaster@example.com> 
250 ok 
If the server does not accept the request, the client quits. 
The client then sends one RCPT request for each envelope recipient address, waiting for a response  
after each address: 
RCPT TO:<support@example.com> 
250 ok 
The client keeps track of which addresses are accepted. If none of the addresses are accepted, the 
client quits. 
The client then sends a DATA request, and waits for the response: 
DATA 
354 go ahead 
If the server does not accept the request, the client quits. 
The client then sends the encoded message, waits for the response, and quits: 
Date: 8 Aug 1998 04:10:45 -0000 
From: postmaster@example.com 
To: support@example.com 
Subject: chariot is back in service 
I patched up the wheel. It's as good as new. 
. 
250 ok 902549473 qp 24035 
QUIT 
221 example.com 
If the client encounters a message I/O error, it closes the connection immediately, without sending 
a final dot and without sending a quite request. On sending the final dot, the handshake is 
complete and the mail server queues the mail for further processing.

Example with SMTP authentication

The following command opens a TCP connection from the sending machine to the mail server listening on port 25.

Open a telnet tool on your mail server.
On most PC you can open a dos window and:
 Enter "telnet"
 Enter "set LOCAL_ECHO"
 Enter "open mail.YourDomain.com 25" (replace YourDomain.com with your domain)
 Enter "HELO"
 Enter "AUTH LOGIN"
You now need to enter your emailid encoded in BASE64, press ENTER and then your password encoded in BASE64.
For help converting your email and password to Base64 click http://www.motobit.com/util/base64-decoder-encoder.asp
Enter your email and click "Encode" copy that value in the telnet session and  press ENTER
Now enter you password in the conversion tool  and copy that value in the  telnet session and press ENTER
If you have successfully authenticated the server will return: "235  authenticated"
 Enter "MAIL FROM YourUserName@YourDomain.com"  (replace  YourUserName@YourDomain.com   with your  email address)
 Enter  "RCPT TO  DestinationEmail"   (replace DestinationEmail  with the destination email)
 Enter "DATA"
 Enter "Subject: Test using telnet"
 Enter ""  (Send  a blank line to separate the headers from the message body)
 Enter "This is just a test to see if I can SMTP auth  from my PC"
 Enter "."