When Exchange have to send a NDR message it will by default try to detect the language of the received message and send the NDR back in the same language.

Sometimes it will not be able to detect the language of the received message and therefore sends the NDR in the language that is default for the configuration.

To set the default NDR language you have to run a PowerShell command.

Use the following command to set the default language to English for NDRs

Exchange 2007

NDRs for mails received from external domains:

Set-TransportServer <hubservername> -ExternalDsnDefaultLanguage en-us More...