per domain outgoing relay host + credentials

Where do I input the credentials for the smarthost server?
And is it possible to use different smarthost for sender domain?
For example:
My mail server hosts 2 domains, and it uses proxmox as the smarthost
And proxmox would use smarthost.example2.com:465 for messages sent from @domain1.com, and smarthost.example2.com:465 for messages sent from @domain2.com

Is any of this possible?
I know that postfix can do this, but PMG overrides any custom configuration in those config files.