I've restricted inbound SMTP connections on port 25 of my firewall to allow only e-mail from 8 IP addresses of a spam filter service outside the firm.
How do I send a test e-mail message directly to the external static address of my firewall to see if e-mail is delivered outside the range of 8 IPs?
There are a couple of ways you can do this. One way would be to go to a computer that is not on the list and telnet to port 25 on your SMTP Server. Then, issue some mail commands to send a test message and see if it is rejected or delivered. If you have not used Telnet to test an SMTP Server before, you can find details on how to do this at http://support.microsoft.com/default.aspx?scid=kb;en-us;153119
Another way to test this is to use a mail client such as Outlook Express from a system that is not on the list. Set the SMTP Server in the mail client to your SMTP Server and once again send a message. If the message is rejected, your configuration is likely working as expected. If the message is delivered, you'll want to double-check your settings.
This was first published in April 2004