After deploying the Symantec Messaging Gateway (SMG), All Email to one or more domains you are hosting, or Email you are sending to one or more external domains is rejected by the SMG with a reported error "554 Recipient address rejected: User unknown".
The sender of an Email received an NDR notification from your Messaging Gateway with the following content :
Remote host said: 554 Recipient address rejected: User unknown
There are two known reasons why this behavior can occur depending on the direction of mail affected including: For issues receiving Email to the Messaging Gateway from external domains: This can happen if the recipient domain has not been added to the "Domains" configuration of the SMG Control Center, or if the domain name has been entered incorrectly. For issues sending Email via the Messaging Gateway to external domains: This can happen if your downstream Mail Transfer Agent (MTA) is sending Email to the Messaging Gateway on the Inbound interface instead of the Outbound interface.
For issues receiving Email to the Messaging Gateway from external domains:
Ensure that the recipient domain, is correctly entered into the Control Center Protocols->Domains page.
For further information on the Local Domains configuration, please consult the Symantec Messaging Gateway Administration Guide at the following location:
For issues sending Email via the Messaging Gateway to external domains:
Ensure that your downstream MTA's SMTP Connector/Smarthost is connecting to the Outbound Interface IP/port specified in the Messaging Gateway's SMTP configuration.
If you are using Messaging Gateway version 8.0.3 or prior, with version 9.0.X, you can now use the same IP address and port for both inbound messages and outbound messages potentially avoiding this issue from occuring. With version 9.0.X, the Scanner will check the outbound mail acceptance settings to determine if a message is inbound or outbound.