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Trouble receiving mail from outside a Checkpoint firewall

Scott, I have an Exchange2k/SP3 box behind a Checkpoint firewall. I have three external mail sites that cannot send to me. I receive mail from other outside mail domains, but can't figure out why these three cannot send me mail. The send gets a Non-Delivery Report (NDR). The firewall is configured to accept mail from everyone. The only sites I have configured to block are some Korean servers from which I was getting hit with spam. Any ideas where to start checking?
The first thing to check is the NDR itself. It will tell you specifically (albeit sometimes cryptically) why the message could not be delivered. (That's why it is called a Non-Delivery Report.) You may need to verify the presence of your Mail Exchanger (MX) record by using the NSLookup utility. Without knowing exactly what NDR message you received, it's difficult to give you specific steps for troubleshooting this. If you have difficulty translating the NDR code, you can look them up at Technet and Microsoft support.

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