A new employee started with the company. Someone setup their AD account incorrectly by spelling their last name wrong (ex. jsmyth@contoso.com). Instead of "renaming" the properties of the existing account, that person chose to create a new AD account with the correct name spelling (ex. jsmith@contoso.com). They deleted the incorrect AD account. This created an "unowned" mailbox on the server. I have since purged the mailbox from the server but found an interesting problem. If I telnet into our SMTP port and try to send mail to that incorrect address (jsmyth@contoso.com), the server is responding with a "250 Ok"? I'm confused as to why the server isn't responding with a "550 not a valid mailbox" error? Also, if I send mail to jsmyth@contoso.com....I never get a 'System Administrator: Undeliverable" message...

I believe that when you telnet, the server will accept the address even if it is miss-spelled during the telnet session. (someone can correct me if i am wrong). Are you sure that the other mailbox "jsmyth" was deleted? NDR should go back to the From address.

Are you sure the computer you're doing the telent to exchange doesnt have relay relay rights on a receive connector that has externally secured if so it will bypass all anti spam agents. I'm sure you can can telnet and enter another dummy address and it will also accept it as well.James Chong MCITP | EA | EMA; MCSE | M+, S+ Security+, Project+, ITIL msexchangetips.blogspot.com

Did you follow the document to have a try? In my test, no need to restart the services. Thanks, EvanEvan Liu TechNet Community Support

Jamestechman and Evan, I believe you are both on the right track. I tried an email address I know doesn't exist and it came back the same, "250 OK". When I have some more time later today, I will do some further research into the relay rights on the receive connector as Jamestechman has suggested. Thank you all for your replies. Please follow this document to check on your Exchange Server: Configure Exchange 2003 to check recipients in SMTP protocol http://blogs.technet.com/b/dlemson/archive/2003/10/17/52019.aspx If you have not enabled that, please follow it to configure on your Exchange 2003. Thanks, EvanEvan Liu TechNet Community Support

What is the result when you check for other inexistence users? If this issue not occur on other users, I suggest you restart services to have a try or test some time later. This may your change have not updated on the server. Thanks, EvanEvan Liu TechNet Community Support

Recipient filtering is not enabled...

Will this require me to restart any Exchange services? Or should I see the change pretty quickly?

Jamestechman and Evan, I believe you are both on the right track. I tried an email address I know doesn't exist and it came back the same, "250 OK". When I have some more time later today, I will do some further research into the relay rights on the receive connector as Jamestechman has suggested. Thank you all for your replies.

Looks like you have to restart the msexchangetransport service.James Chong MCITP | EA | EMA; MCSE | M+, S+ Security+, Project+, ITIL msexchangetips.blogspot.com

That is the problem once you enable it will will start issuing user unknown.James Chong MCITP | EA | EMA; MCSE | M+, S+ Security+, Project+, ITIL msexchangetips.blogspot.com

I checked our connectors and we don't have any since there is only one mail server for our company. Also, I tried the telnet test from my home PC and I am still getting "250 Ok" when inputting any incorrect mail address (dummy address). We do have a third party spam filter, could it be causing this symptom? Why do I still get "250 Ok" even when testing dummy addresses?

It could be by design, you need to check your spam filter settings. Some spam filters settings will just accept all email addresses so that spammers cannot directory harvest your email addresses.James Chong MCITP | EA | EMA; MCSE | M+, S+ Security+, Project+, ITIL msexchangetips.blogspot.com

Would this behavior occur if we had NDR turned off? "Allow non-delivery reports" is unchecked on our server. I learned that we did this on purpose (before I arrived). Do you think this would directly cause the symptom I'm experiencing?

No disabling NDR is not involved, can you see if you have recipient filtering enabled? http://www.msexchange.org/tutorials/sender-recipient-filtering.html James Chong MCITP | EA | EMA; MCSE | M+, S+ Security+, Project+, ITIL msexchangetips.blogspot.com

Evan, I read the documentation that you provided. Thank you. I have to get with my team before I can implement any changes and we are all really busy at the moment. I will post back if\when I am able to make the change.