The MTA hostname you choose for your Mailborder server cannot be the same as any other server. It must also have valid DNS records and match your MX records.
It is not uncommon to see administrators name their internal email server something like mail.domain.com and then use that same name on their Mailborder server's MTA hostname. This will not work. Mailborder will detect that your internal server is using the same hostname and email will not be delivered.