How can I stop someone from using my server to send spam?

I think my Domino server in being is used by someone to send out spam (as an open relay). Can you give me steps to follow to stop the process and any advice you think we should follow? I am new to Domino 6.
This problem is a little complex to address here. I will give you some basics.

Download this free guide

Download: IT certifications that stand out

Are you looking to advance your career? Expand your knowledge? Boost your credibility? Our editors put together this complimentary 19-page guide on everything you need to know about obtaining an IT certification—with special attention given to cloud and desktop certifications.

Start Download

• Corporate E-mail Address:

  You forgot to provide an Email Address.

  This email address doesn’t appear to be valid.

  This email address is already registered. Please login.

  You have exceeded the maximum character limit.

  Please provide a Corporate E-mail Address.

• • I agree to TechTarget’s Terms of Use, Privacy Policy, and the transfer of my information to the United States for processing to provide me with relevant information as described in our Privacy Policy.

  Please check the box if you want to proceed.

• • I agree to my information being processed by TechTarget and its Partners to contact me via phone, email, or other means regarding information relevant to my professional interests. I may unsubscribe at any time.

  Please check the box if you want to proceed.

By submitting my Email address I confirm that I have read and accepted the Terms of Use and Declaration of Consent.

First, you should restrict inbound relayers and outbound relayers in the configuration document to only the domains you manage. You also should set the notes.ini parameter SMTPAllHostsExternal=1. This parameter requires an OS reboot (not just a Domino restart) to take effect. There are a myriad of other settings to make your setup effective in the configuration document. Unfortunately there isn't the time or space to mention them here. I am actually writing an article on this exact topic, and it is currently coming in at about 25 pages. So, you can imagine how complex this task can be.