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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.abuse.net/relay.html
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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