POSTFIX> add exception to not rewrite sender

Hi,

We have a domain all mail are managed by outlook.com (with office 360).

In virtualmin, we had desactivated mail function, everything are good,
except for mail sent by php are rewrited by thzmachine name.

contact@mydomain.com rewrited to contact@ip-server-name.

My question how can we add an exception to not rewrite domain name ?

Thank you