Mail-Abuse.org
--------------
telnet relay-test.mail-abuse.org
runs an open relay test!

OR

Open Relay Test using web form
------------------------------
http://members.iinet.net.au/~remmie/relay/

OR

SMTP Commands to test for open relay
------------------------------------

telnet smtp.example.com 25 (from remote location)

EHLO hostname

MAIL FROM: user@example.com

RCPT TO: user@otherexample.net

(should fail here) Relaying denied.

DATA

<type mail headers>
subject: foo

end message with a period (.)

then issue the command QUIT

OR

From Windows PowerShell
-----------------------

Send-MailMessage -To user@otherexample.net -From user@example.com -Subject test -Body test -SmtpServer fqdnofmailserverht.example.com

fqdnofmailserverht = FQDN of your organization's Hub Transport server