How to check Whether your Exchange Server is an Open SMTP Relay
To do this we will use Telnet which involves establishing a session from a computer that is not on the same network as the external (public) IP address of the Exchange server. Performing this from a machine on your own network will not produce accurate results.
1. Open a command prompt.
2. Type "telnet" and press enter.
3. At the Telnet prompt, type the following and press enter
4. Type the following and press enter
open mail.mydomain.com 25
where mail.mydomain.com is the address of your Exchange server. You should receive a response similar to this:
Quote:
220 mail.mydomain.com Microsoft ESMTP MAIL Service ready at Thu, 20 Aug 2009 21:06:02 +0100
5. Type the following and press enter
where testdomain.com is a domain not hosted on your Exchange server. You should receive a response similar to this:
Quote:
250 OK
5. Type the following and press enter.
mail from:someone@testdomain.com
You should receive a response similar to this:
Quote:
250 2.1.0
address@testdomain.com....Sender OK
5. Type the following and press enter.
rcpt to:someone@anotherdomain.com
where anotherdomain.com is a domain also not hosted on your Exchange server. You should receive a response similar to this:
Quote:
550 5.7.1 Unable to relay
This means your server is not an open relay. However, if you receive a response similar to this:
Quote:
Then your server is an open relay.
