How to check SMTP Relay is Open or not

Telnet to mail Server - testmail.mailserver.com on 25 port

C:\telnet {testmail.mailserver.com} 25

You will get result like this

220 testmail.mailserver.com Microsoft ESMTP Mail Service Verions:6.0.3790.1830 Ready At {Date and time}

Put the following command to test relay is open or not

Helo
250 testmail.mailserver.com Hello {Your Machine IP}
Mail From: Test@mailserver.com
250 2.1.0 Test@mailserver.com..... Sender OK
RCPT To:testmail@gmail.com
550 5.7.1 Unable to Relay for testmail@gmail.com

This indicate that Mailserver is not open for Relaying email for external domain.