Thousands of "Undelivered Mail Returned to Sender" messages everyday from mail I did not send!

I'm running Exchange 2003 on W2k3 server. Lately, I'm getting literally thousands of returned mail messages daily as if someone is sending mail from my domain to huge lists and I'm getting all of the returned mail.  Is there anyway to tell whats going on here?

I started digging around in my Exchange System Manager and found a bunch of unknown domain names in my outbound SMTP connections.  I frooze all of them besides my domains.  How can I stop this?!?!
MFredinAsked:
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moorhouselondonCommented:
This problem is known as Backscatter.

http://spamlinks.net/prevent-secure-backscatter.htm
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MarcBorstCommented:
Someone is faking your email-address. Of course, the undeliverable report returns to the correct address. First of all please check, if your server is an open smtp-relay. Please post the settings of the follwing here.

System Manager > Administrative Groups > First Administrative Group > Servername > Protocols > SMTP > Virtual Standard Server for SMTP > Properties > Tab "Access" > Relay.

Maybe Relaying is allowed for everyone. If you search the internet about "testing open smtp relay" you will also find some websites, where you can do that.
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MFredinAuthor Commented:
Under relay I have "Select which computers may relay through this virtual server: ONLY THE LIST BELOW"  and I dont have any computers in the list.  I also have "Allow all computers which successfully authenticate to relay, regardless of the list above" CHECKED.

I did a few relay tests and I am NOT an open SMTP relay.
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