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Why is my Email Security Gateway blocking mail with a reason of "Invalid Domain"?

  • Type: Knowledgebase
  • Date changed: 3 years ago
Solution #00003608

All Email Security Gateways, all firmware versions.

The Email Security Gateway will reject emails for Invalid Domain when they are addressed to recipients at a domain that has not been configured on the Email Security Gateway, either at the bottom of the Basic > IP Configuration page or under the Domains tab. 

If legitimate email is being blocked for this reason, add your domains to the Email Security Gateway on the Domains > Domain Manager page.

Additional Notes:
If you are using an inbound Email Security Gateway as an outbound relay and your outgoing mails are being rejected for Invalid Domain, most likely the IP address(es) of the sending mail server(s) have not been entered properly on the Advanced > Outbound / Relay page, or they may have changed since they were configured. In firmware 4.0+, this is located at BASIC > Outbound - Relay Using Trusted IP/Range.

If a mail server is trying to send outgoing mail through an inbound Email Security Gateway and the Barracuda has not been set up to relay that mail, the Email Security Gateway will treat that mail as incoming mail and block it with a reason of Invalid Domain because the outgoing recipient domains, like and, have not been configured on the Barracuda to receive mail. For more help on setting up Outbound Relay, see Solution #00002087.

If you see mail being blocked for this reason and you are certain the domain has been configured properly, please call Barracuda Networks Technical Support.

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