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Recently our domain was flagged as spam, and we found out that emails with spam are being sent from inside our organization but with domain different than our Domain.

Example: our domain : email@example.com the spam email sender: spamer@imf.com

so am not sure how this happened

any help would be appreciated, thank you

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In Internet Information Services (IIS) 6.0 Manager, find the name of your server and expand it and then select [SMTP Virtual Server #1], right click it and select Properties. A window opens. On the Access tab click the "Relay" button and another window opens. On this window I suspect you have a blob in the hole for "all except the list below" and you need to change that to put a blob in the hole for "Only the list below". Then you can add the IP address of your mail server and any other machines on your network that should be allowed to send relayed emails. You can also put a tick in the box for allowing computers that authenticate to relay too. Then click OK on any open windows and close the IIS windows and you should be good to go. Obviously note what you have done and test.

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You have an open relay somewhere. SMTP does not check for valid FROM lines protocol wise (which is why there is a spam problem today). Check the header of a spam mail, the sending IP should be there in the first RECEIVED line.

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