How to set up mail forwarding from hosting to local mail server iredmail?

Question

postfix

Good afternoon.
I ask for help on this issue:
Hosting has a domain with mailboxes. Iredmail (Postfix, Dovecot, Amavisd, ClamAV, SpamAssassin, RoundCube) is deployed on the local network. In a network mail goes normally. How can I make sure that letters first come to the provider's mail server, and then are pumped out by the existing iredmail mail server (mydomen.ru->mydomen.local). So that in case of unavailability of the internal corporate mail server, letters are stored on the hosting. And also so that users can send mail outside using the authorization data of the hosting domain (mydomen.local ->mydomen.ru).