I assume this means you have an MX record at example.com pointing to your/mailbox.org SMTP server? AIUI, a sending MTA will look up the MX record for example.com by preference order and will deliver emails to the first server that accepts the connection.
So it may depend if you can configure your mailbox.org account/server to reject connections from servers trying to send mail to unknown addresses? Then the sending MTA server might "fail over" to the Relay server instead?
So it may depend if you can configure your mailbox.org account/server to reject connections from servers trying to send mail to unknown addresses? Then the sending MTA server might "fail over" to the Relay server instead?