I would like to configure my ISPConfig server to provide backup MX to a domain that is not part of my ISPConfig setup. If I try to do it using an "Email transport" setup, I get complaints about "Unknown mailbox". That had me baffled for ages but now I understand that ISPConfig really only expects you to be doing this for domains within your ISPConfig world. Is there a way to get ISPConfig to "just relay a domain without worrying about mailbox existence"? If not, can I manually manipulate the Postfix setup without confusing ISPConfig? Thanks.
You can setup a email relay with ispconfig. I guess that the error you describe happens because you missed to add the email addresses or email domains that shall be part of the backup mx under global filters > relay recipients.
