Microsoft has a good article on testing an SMTP server using Telnet.  I’ve summarized the steps here using the following information:

SMTP server name mail.contoso.com
Source domain name test.com
From email address Admin@test.com
Recipient email address User@domain.com

Here is the summary of commands and responses.  Remember to press <ENTER> after each command.

Command Response
telnet mail.contoso.com 25 220 site.contoso.com Microsoft Exchange Internet Mail Connector <version number of the IMC>
EHLO test.com 250 OK
MAIL FROM:Admin@test.com 250 OK - MAIL FROM Admin@test.com
RCPT TO:User@domain.com 250 OK - Recipient User@domain.com
DATA 354 Send data. End with CRLF.CRLF
Subject: test message  
<ENTER>  
This is a test message.  You will not see a response from this command.  
. 250 OK
QUIT 221 closing connection