Sadly your use-case is dying. Asymmetric mail flows are becoming harder and harder to support, as DKIM/DMARC alignment and SPF become stronger anti-spam signal

Sadly your use-case is dying. Asymmetric mail flows are becoming harder and harder to support, as DKIM/DMARC alignment and SPF become stronger anti-spam signals, we're going to have to lock down on egress for domains which we don't have proof of control for.