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No NDR for invalid addresses on specific domain

Posted on 2011-05-11
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Our company is having issues receiving proper NDR's from invalid e-mail addresses on a specific domain. For example, using a fake address like gobbledygook@outsidedomain.com will not send a reply to us that the address is invalid. This works sending from other e-mail servers such as Gmail and Hotmail.

Looking at our message tracking center in Exchange 2003 shows the test messages being transferred to what looks like a proxy on their end (they are a Taiwan domain), but the message tracker ends there.
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Question by:dcgagne
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Brian B earned 2000 total points
ID: 35738837
Most likely the receiving domain is set not to send NDR. This is one way of preventing a dictionary attack, but is not a particularly good one since it ignores RFC conventions.

If you want to see what is actually happening try a manual telnet session on port 25 to the recipient's MX server.
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by:dcgagne
ID: 35738893
Unfortunately, the external domain has telnet access blocke, so testing to them is limited. If it helps, here is the error message from a gmail box when trying to send to that non-existent address:

Delivery to the following recipient failed permanently:

    [e-mail address redacted]

Technical details of permanent failure:
Google tried to deliver your message, but it was rejected by the recipient domain. We recommend contacting the other email provider for further information about the cause of this error. The error that the other server returned was: 500 500 5.0.0 Service unavailable (state 18).
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by:Brian B
ID: 35738956
You can't block telnet per se, you block the port that telnet uses. In the case of SMTP it is 25. So if you can't reach them on port 25, there is your problem. Their server doesn't conform to standards, therefore you can't communicate with it.

The GMail error is a similar story. The service on the reipient's server is not working.

No matter what, I am sure this has nothing to do with your system.
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Author Comment

by:dcgagne
ID: 35738979
Thanks, I was sure we were fine on our end but I needed to cover all my bases.
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