External MTA (while DNSSEC enabled)

Case: “Offshore” MTA is taking care (only) of anti virus and (additional) spam check (stronger outcome/fine Tuning possible than spamassassin on MiaB), both having the same IP as the MiaB-Box.

Question: How can we set up an external MTA (while DNSSEC on MiaB is enabled!) to avoid loop backs when sending mails? One cannot use the same hostname on both MTAs (e.g. box.mailinabox.mail), otherwise this will result in loop backs when sending mails.