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Preventing 'open relay' on SMTP servers

My network is connected to the Internet via T1. Our system has been working fine for months. Now, all of a sudden, we can no longer send e-mail or access the Internet. But strangely enough, we are still receiving e-mail. E-mail is hosted at my ISP. What could be causing this problem? (Pinging an external public IP address works.)

Without knowing the details of your SMTP configuration, it sounds like your SMTP server was detected as being an open relay and you were added to a blacklist. You can check the status of your SMTP server at the following URLs:

http://www.dnsstuff.com/ (spam database lookup)

If your SMTP server is detected as being an open relay, you will need to follow your vendor procedures for configuring your SMTP server to only relay mail for IP addresses on your internal network. In addition, here are some resources for Microsoft Exchange servers for preventing your server from being used as an open relay:

Advice from Eric Harrison, server administration expert
Microsoft TechNet's top 4 exchange server security best practices

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