Why do I get a "451 unresolvable host name" error message?

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When Sending mail to some servers, I see an error message in the logs which says:

451 <>... unresolvable host name

What is the cause of this?


RFC2821 States that mail responses should be sent with a null reverse path – ie:


Some mail servers allow a setting whereby addresses can be looked up in DNS to verify that they are valid. In certain implementations that ignore Internet standards, this results in mail from the null address – "<>" – being rejected. Since GMS complies with the RFC and sends the mail with this from address, this results in a problem. Receiving servers should not be performing lookups on the null address because they will then refuse return mail from any RFC2821 compliant mail server.

Using GMS Anti-Spam, you can configure reverse lookups for sending addresses, however the null address is handled correctly.

The problem lies in the non RFC compliant receiving server, you should contact the administrator of the server in question to notify them of their problem.

This will also effect mail from the following:

Receipts (Open, Read, Acknowledge)
Error reports

All Gordano products are fully RFC compliant.

Keywords:451, unresolvable, hostname, error message

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